What's this going on and if you need it to expand to 3 inches, I assume the pipes that are there are currently 3 inches in diameter?
You might be over thinking this. If you need to mate 2.5 up to 3, you can simply get some reducers. Reducers are made for this sort of thing, where you either need to increase or decrease the size by 1/2 and inch. I used reducers to help splice in my Corsa muffler with its 2.5 inch inlet/outlets to the factory 2.75 pipe diameter. Worked perfectly and was a cheap solution.