It would help to mention a bit more detail about the originating computer, the Cobol version, and the type of file (in Cobol terms), and the type of record (in Cobol terms), the file transfer method and any subsequent conversion if any was done (e.g. EBCDIC to ASCII).
Anyway. Google Advance Search "Cobol AIX Microfocus conversion" with exact match "Comp-3" produced many hits including this one which mentions your exact error message and discusses the issue:
AIX Cobol II and Packed Decimal fields - Rhinocerus
Google Advance Search "Cobol AIX Micro Focus conversion" with exact match "Comp-3" produced even more interesting hits. It will mean much more to you armed with knowledge of the file and record format.
If you have a licenced copy of Microfocus Cobol it may well be easier to ask Microfocus.
(I'm sure it was a typo in your post, but if you are Googling make sure you spell "Microfocus" or "Micro Focus" correctly
Application Modernization and Management - Micro Focus).