Is it OK to replace a TRANE 3 ton condenser/compressor with a Goodman unit?

Yes but you need to make sure the coil,refrigerant,and piston/TX valve is all correct. I would suggest also replacing the inside coil also.

Yep,, in most cases when you replace the condenser unit it is going to be higher seer than your old unit,and in order to be under warranty you MUST put in a bigger indoor coil