The diameter of the pipe has very little effect on the sound volume.

A 3" pipe will, however, make you lose bottom end torque. Suggest you go with the 2.5" pipe from cat back (IF you even need to replace the whole thing, which is doubtful). Then use a single 2.5" in, dual 2" out muffler. You can always use larger dia. tips, if that'* what you're looking for.

i am going use the same design that they use on a truck except the striaight pipe will be i the middle