Exhibit 99.3


         INSTRUCTIONS TO REGISTERED HOLDER AND/OR BOOK-ENTRY TRANSFER
                  FACILITY PARTICIPANT FROM BENEFICIAL OWNER

                                      OF

                        AMSC ACQUISITION COMPANY, INC.

                    SERIES A 12 1/4% SENIOR NOTES DUE 2008

To Registered Holder and/or Participant of the Book-Entry Transfer Facility:

         The undersigned hereby acknowledges receipt of the Prospectus, dated
May __, 1998 (the "Prospectus"), of AMSC Acquisition Company, Inc., a Delaware
corporation (the "Company") and American Mobile Satellite Corporation, a
Delaware corporation and direct parent of the Company ("Holdings") and the
accompanying Letter of Transmittal (the "Letter of Transmittal"), that together
constitute the Company's offer (the "Exchange Offer"). Capitalized terms used
but not defined herein have the meanings ascribed to them in the Prospectus.

         This will instruct you, the registered holder and/or book-entry
transfer facility participant, as to action to be taken by you relating to the
Exchange Offer with respect to the 12 1/4% Series A Senior Notes due 2008 (the
"Old Notes") held by you for the account of the undersigned.

         The aggregate face amount of the Old Notes held by you for the account
of the undersigned is (FILL IN AMOUNT):

         $ ___________________ of the 12 1/4% Series A Senior Notes due 2008

         With respect to the Exchange Offer, the undersigned hereby instructs
you (CHECK APPROPRIATE BOX):

         [_] TO TENDER the following Old Notes held by you for the account of
the undersigned (INSERT PRINCIPAL AMOUNT OF NOTES TO BE TENDERED, IF ANY):
$____________.

         [_] NOT TO TENDER any Old Notes held by you for the account of the
undersigned.

         If the undersigned instructs you to tender the Old Notes held by you
for the account of the undersigned, it is understood that you are authorized (a)
to make, on behalf of the undersigned (and the undersigned, by its signature
below, hereby makes to you), the representations and warranties contained in the
Letter of Transmittal that are to be made with respect to the undersigned as a
beneficial owner, including but not limited to the representations that (i) the
undersigned is acquiring the Exchange Notes in the 

 
ordinary course of business of the undersigned, (ii) the undersigned is not
participating, does not participate, and has no arrangement or understanding
with any person to participate in the distribution of the Exchange Notes, (iii)
the undersigned acknowledges that any person participating in the Exchange Offer
for the purpose of distributing the Exchange Notes must comply with the
registration and prospectus delivery requirements of the Securities Act of 1933,
as amended (the "Act"), in connection with a secondary resale transaction of the
Exchange Notes acquired by such person and cannot rely on the position of the
Staff of the Securities and Exchange Commission set forth in no-action letters
that are discussed in the section of the Prospectus entitled "The Exchange 
Offer --Resales of the Exchange Notes," and (iv) the undersigned is not an
"affiliate," as defined in Rule 405 under the Act, of the Company; (b) to agree,
on behalf of the undersigned, as set forth in the Letter of Transmittal; and (c)
to take such other action as necessary under the Prospectus or the Letter of
Transmittal to effect the valid tender of such Old Notes.

                                   SIGN HERE

Name of Beneficial 
Owner(s):
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Signature(s):
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Names (please 
print):
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Address: 

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Telephone 
Number:
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Taxpayer Identification or Social Security Number:
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Date:  
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