I've never read or heard about this situation, have you got more details? Version of 3.0 that created it? (3.0 or 3.0b?) Service Pack level of 6.0 runtime that can't "open" it? (Original, SP 1, 2, 3, 4, 5 and/or 6?) What error message(s) do you get? Is it a specific field type or feature that causes a problem?
There is a little problem. If dbf file is in dbc created by VFP3, it can be opened by VFP6 development environment, but not by VFP6 runtime
There is no need to "convert" a 3.0 .DBF to 6.0. While VFP 5/6/7/8 .DBFs and .DBCs have additional features, they all have the same structure. If the original tables are "free" you could add them to a database (.DBC) if that's what you have in mind.