<PAGE>

                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C.  20549


                                  SCHEDULE 13G


            INFORMATION STATEMENT PURSUANT TO RULES 13d-1 AND 13d-2
                  UNDER THE SECURITIES AND EXCHANGE ACT OF 1934
                               (Amendment No. 3)*



                                  Unisys Corp.
- --------------------------------------------------------------------------------
                                (Name of Issuer)


                                  Common Stock
- --------------------------------------------------------------------------------
                         (Title of Class of Securities)


                                    909214108
- --------------------------------------------------------------------------------
                                 (CUSIP Number)



Check the following box if a fee is being paid with this statement [ ].  (A fee
is not required only if the filing person:  (1) has a previous statement on file
reporting beneficial ownership of more than five percent of the class of
securities described in Item 1; and (2) has filed no amendment subsequent
thereto reporting beneficial ownership of five percent or less of such class.)
(See Rule 13d-7.)

____________________

*The remainder of this cover page shall be filled out for a reporting person's
initial filing on this form with respect to the subject class of securities, and
for any subsequent amendment containing information which would alter
disclosures provided in a prior cover page.

The information required on the remainder of this cover page shall not be deemed
to be "filed" for the purpose of Section 18 of the Securities Exchange Act of
1934 ("Act") or otherwise subject to the liabilities of that section of the Act
but shall be subject to all other provisions of the Act (however, see the
Notes).


                        (Continued on following page(s))
                                Page 1 of 8 Pages



<PAGE>

  CUSIP No. 909214108                  13G                     Page 2 of 8 Pages


 1   NAME OF REPORTING PERSON
     S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON

          The TCW Group, Inc.  04-2254452


 2   CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*
                                                                    (a) /   /
                                                                    (b) / X /

 3   SEC USE ONLY


 4   CITIZENSHIP OR PLACE OF ORGANIZATION

          Nevada corporation

   NUMBER OF         5   SOLE VOTING POWER
    SHARES                                                         10,354,912
 BENEFICIALLY
   OWNED BY          6   SHARED VOTING POWER
     EACH                                                                 -0-
   REPORTING
    PERSON           7   SOLE DISPOSITIVE POWER
     WITH                                                          10,354,912

                     8   SHARED DISPOSITIVE POWER
                                                                          -0-

 9   AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
                                                                   10,354,912

10   CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES*
                                                                        /   /

11   PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9)
      5.9%(see response to Item 4)

12   TYPE OF REPORTING PERSON*

                                      HC/CO


                      *SEE INSTRUCTIONS BEFORE FILLING OUT!


<PAGE>

  CUSIP No. 909214108                  13G                     Page 3 of 8 Pages


 1   NAME OF REPORTING PERSON
     S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON

          Robert Day


 2   CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*
                                                                    (a) /   /
                                                                    (b) / X /

 3   SEC USE ONLY


 4   CITIZENSHIP OR PLACE OF ORGANIZATION

          United States Citizen


   NUMBER OF         5   SOLE VOTING POWER
    SHARES                                                         10,354,912
 BENEFICIALLY
   OWNED BY          6   SHARED VOTING POWER
     EACH                                                                 -0-
   REPORTING
    PERSON           7   SOLE DISPOSITIVE POWER
     WITH                                                          10,354,912

                     8   SHARED DISPOSITIVE POWER
                                                                          -0-


 9   AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
                                                                   10,354,912

10   CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES*
                                                                        /   /

11   PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9)
       5.9% (see response to Item 4)

12   TYPE OF REPORTING PERSON*7

                                      HC/IN


                      *SEE INSTRUCTIONS BEFORE FILLING OUT!


<PAGE>

                                                               Page 4 of 8 Pages

Item 1(a).   Name of Issuer:

             Unisys Corp.

Item 1(b).   Address of Issuer's Principal Executive Offices:

             P.O. Box 500
             Blue Bell, PA 19424

Item 2(a).   Name of Persons Filing:
Item 2(b).   Address of Principal Business Office:
Item 2(c).   Citizenship:

             The TCW Group, Inc.
             865 South Figueroa Street
             Los Angeles, CA 90017
             (Nevada Corporation)

             Robert Day
             200 Park Avenue, Suite 2200
             New York, New York 10166
             (United States Citizen)

Item 2(d).   Title of Class of Securities:

             Common Stock

Item 2(e).   CUSIP Number:

             909214108



<PAGE>

                                                               Page 5 of 8 Pages

Item 3.   If this statement is filed pursuant to Rules 13d-1(b), or 13d-2(b),
          check whether the person filing is a:

     (a)  [ ]  Broker or Dealer registered under Section 15 of the Act:

                    Not applicable

     (b)  [ ]  Bank as defined in Section 3(a)(6) of the Act:

                    Not applicable

     (c)  [ ]  Insurance Company as defined in Section 3(a)(19) of the Act:

                    Not applicable

     (d)  [ ]  Investment Company registered under Section 8 of the Investment
               Company Act:

                    Not applicable

     (e)  [ ]  Investment Adviser registered under Section 203 of the Investment
               Advisers Act of 1940:

                    Not applicable

     (f)  [ ]  Employee Benefit Plan, Pension Fund which is subject to the
               provisions of the Employee Retirement Income Security Act of 1974
               or Endowment Fund (SEE 13d-1(b)(1)(ii)(F)):

                    Not applicable

     (g)  [X]  Parent Holding Company, in accordance with Rule 13d-1(b)(ii)(G)
               (SEE Item 7):

                    The TCW Group, Inc.
                    Robert Day (individual who may be deemed to control The TCW
                         Group, Inc. and other holders of the Common Stock of
                         the issuer)

     (h)  [ ]  Group, in accordance with Rule 13d-1(b)(1)(ii)(H):

                    Not applicable.


<PAGE>

                                                               Page 6 of 8 Pages

Item 4.   Ownership **

     THE TCW GROUP, INC.

          (a)  Amount beneficially owned: 10,354,912

          (b)  Percent of class: 5.9%

          (c)  Number of shares as to which such person has:

               (i)   Sole power to vote or to direct the vote: 10,354,912

               (ii)  Shared power to vote or to direct the vote: none.

               (iii) Sole power to dispose or direct the disposition of:
                     10,354,912

               (iv)  Shared power to dispose or to direct the disposition of:
                     none.

     ROBERT DAY

          (a)  Amount beneficially owned: 10,354,912

          (b)  Percent of class: 5.9%

          (c)  Number of shares as to which such person has:

               (i)   Sole power to vote or to direct the vote: 10,354,912

               (ii)  Shared power to vote or to direct the vote: none.

               (iii) Sole power to dispose or direct the disposition of:
                     10,354,912

               (iv)  Shared power to dispose or to direct the disposition of:
                     none.


____________________

**  The filing of this Schedule 13G shall not be construed as an admission that
the reporting person or any of its affiliates is, for the purposes of Section
13(d) or 13(g) of the Securities Exchange Act of 1934, the beneficial owner of
any securities covered by this Schedule 13G.  In addition, the filing of this
Schedule 13G shall not be construed as an admission that the reporting person or
any of its affiliates is the beneficial owner of any securities covered by this
Schedule 13G for any other purposes than Section 13(d) of the Securities
Exchange Act of 1934.


<PAGE>

                                                               Page 7 of 8 Pages

Item 5.   Ownership of Five Percent or Less of a Class.

               Not applicable.

Item 6.   Ownership of More than Five Percent on Behalf of Another Person.

          Various persons other than as described in Item 4 have the right to
          receive or the power to direct the receipt of dividends from, or the
          proceeds from the sale of, the Common Stock of Unisys Corp.

Item 7.   Identification and Classification of the Subsidiary Which Acquired the
          Security Being Reported on by the Parent Holding Company.

          SEE Exhibit A.

Item 8.   Identification and Classification of Members of the Group.

          Not applicable. SEE Exhibits A and B.

Item 9.   Notice of Dissolution of Group.

          Not applicable.

Item 10.  Certification.

     Because this statement is filed pursuant to Rule 13d-1(b), the following
     certification is included:


     By signing below I certify that, to the best of my knowledge and belief,
     the securities referred to above were acquired in the ordinary course of
     business and were not acquired for the purpose of and do not have the
     effect of changing or influencing the control of the issuer of such
     securities and were not acquired in connection with or as a participant in
     any transaction having such purpose or effect.



<PAGE>

                                                               Page 8 of 8 Pages

                                    SIGNATURE


          After reasonable inquiry and to the best of my knowledge and belief, I
certify that the information set forth in this statement is true, complete and
correct.


Dated this 12th day of February, 1996.


                                        The TCW Group, Inc.



                                        By:  /s/ Mohan V. Phansalkar
                                             -----------------------
                                             Mohan V. Phansalkar
                                             Vice President


                                        Robert Day



                                        By:  /s/ Mohan V. Phansalkar
                                             -----------------------
                                             Under Power of Attorney dated
                                             January 30, 1996, on File with
                                             Schedule 13G Amendment Number 1
                                             for Matrix Service Co. dated
                                             January 30, 1996.






<PAGE>

                                    EXHIBIT A


                 RELEVANT SUBSIDIARIES OF PARENT HOLDING COMPANY

PART A: TCW ENTITIES

PARENT HOLDING COMPANY:

            The TCW Group, Inc.

            Robert Day (an individual who may be deemed to control The TCW
            Group, Inc.)


RELEVANT SUBSIDIARIES THAT ARE PERSONS DESCRIBED IN RULE 13d-1(b):

     (i)    Trust Company of the West, a California corporation and a bank as
     defined in Section 3(a)(6) of the Securities Exchange Act of 1934.

     (ii)   TCW Asset Management Company, a California corporation and an
     Investment Adviser registered under Section 203 of the Investment Advisers
     Act of 1940.

     (iii)  TCW Funds Management, Inc., a California corporation and an
     Investment Adviser registered under Section 203 of the Investment Advisers
     Act of 1940.

Note:       No Common Stock of Unisys Corp. is held directly by The TCW Group,
            Inc.  Other than the indirect holdings of The TCW Group, Inc. no
            Common Stock of Unisys Corp. is held directly or indirectly by
            Robert Day, an individual who may be deemed to control The TCW
            Group, Inc.

PART B: NON TCW ENTITIES

PARENT HOLDING COMPANY:

            Robert Day (an individual who may be deemed to control the holders
            described below which are not subsidiaries of The TCW Group, Inc.)

RELEVANT SUBSIDIARIES THAT ARE PERSONS DESCRIBED IN
RULE 13d-1(b):

            Oakmont Corporation,
 a California corporation and an Investment
            Adviser registered under Section 203 of the Investment Advisers Act
            of 1940.

            Cypress International Partners Limited, a British Virgin Islands
            corporation and an Investment Adviser registered under Section 203
            of the Investment Advisers Act of 1940.


                                       A-1




<PAGE>

                                    EXHIBIT B

                             JOINT FILING AGREEMENT


            The undersigned acknowledge and agree that the foregoing statement
on Schedule 13G is filed on behalf of each of the undersigned and that all
subsequent amendments to this statement on Schedule 13G shall be filed on behalf
of each of the undersigned without the necessity of filing additional joint
acquisition statements.  The undersigned acknowledge that each shall be
responsible for the timely filing of such amendments, and for the completeness
and accuracy of the information concerning him or it containing therein, but
shall not be responsible for the completeness and accuracy of the information
concerning the other, except to the extent that he or it knows or has reason to
believe that such information is accurate.


Dated: February 12, 1996


                                        By:  /s/ Mohan V. Phansalkar
                                             -----------------------
                                             Mohan V. Phansalkar
                                             Vice President



                                        Robert Day



                                        By:  /s/ Mohan V. Phansalkar
                                             -----------------------
                                             Under Power of Attorney dated
                                             January 30, 1996, on File with
                                             Schedule 13G Amendment Number 1
                                             for Matrix Service Co. dated
                                             January 30, 1996.


                                       B-1