<PAGE>   1
                                                               EXHIBIT 10.10(c)




                             THIRD AMENDMENT TO THE
                      USML, INC. TAX DEFERRED SAVINGS PLAN


            This Third Amendment to the USML, Inc. Tax Deferred Savings Plan is
adopted pursuant to Treasury Regulation 1.401(b)-1(d)(3) and is executed within
91 days following the issuance of a favorable tax-determination letter by the
Internal Revenue Service on the amendment and restatement of the Plan for the
Tax Reform Act of 1986.

            1.      Section 1.27 of the Plan Document.  In the amendment to
Section 1.27 of the Plan by the Equitable Life Assurance Society First
Amendment to the Plan, the reference to Section 1.9 of the Plan shall be
replaced with a reference to Section 1.10 of the Plan.

            2.      Section 4.4(f)(5) of the Plan Document.  In the first
sentence in the second paragraph of Section 4.4(f)(5) of the Plan, the
reference to May 5 shall be replaced with a reference to May 6.

            3.      Section 9.3(b) of the Plan Document.  This first sentence
in Section 9.3(b) of the Plan shall be amended and restated to read as follows:
"This provision shall not apply to the extent a Participant or Beneficiary is
indebted to the Plan as a result of a loan from the Plan."

            4.      Section 11.7(j) of the Plan Document.  Paragraph (j)
(Multiple Use Test) shall be added at the end of Section 11.7(i) of the Plan to
read as follows:

            (j)     Effective with respect to Plan Years specified in Treasury
            regulations, if the high-paid average contribution percentage
            computed under Section 11.6 above exceeds 125% of the lower-paid
            average contribution percentage and if the high-paid average
            deferral percentage computed under Section 11.4 also exceeds 125%
            of the lower-paid average deferral percentage computed under that
            Section (but in each case, the percentages otherwise satisfy the
            test set forth in Section 11.4(a)), then such contributions and
            deferrals shall remain in the Plan only if the aggregate limit of
            the multiple use test set forth in Treasury Regulation Section
            1.401(m)-2(b) (as may be amended from time to time) is met.  If
            such aggregate limit is exceeded, then the Employer matching
            contributions and the Employee after-tax contributions, or the
            before-tax Employee contributions made by or on behalf of Highly
            Compensated Participants shall be reduced pro-rata as the Plan
            Administrator deems necessary in the same manner as set forth in
            Sections 11.5 and 11.7, until such aggregate limit is not exceeded.






<PAGE>   2





            UNIVERSAL STANDARD MEDICAL LABORATORIES, INC. has caused this Third
Amendment to the USML, Inc. Tax Deferred Savings Plan to be executed on _______
________, 1995.



                                        UNIVERSAL STANDARD MEDICAL
                                        LABORATORIES, INC.



                                        By:___________________________________


                                        Its:__________________________________