Are you talking about 3" turndowns? Or 3" flanges (where the x-pipe meets the flowtubes)? Either way, a muffler shop should be able to weld them together somehow.
I see... Not to sure with 3" flanges, but if nothing else, the muffler shop can cut the flanges off completely and weld in a reducer between the x-pipe and the cat-back. Give it a shot, cus I don't think anybody sells 2.5" to 3" flange adapters.