Why 3"
3" have gains on high comp B series and K series builds..plus (of course) boost..
But if your staying with a D, then a 2.5 is fine.
If your planning a build and the 3" will help you, by all means go for it.
Heres a kit which has all hardware and hangers. Pick your resonator and muffler and your good to go.
HONDA TK
( I have twin 18" resonators and a Vibrant muffler and Im still loud when into the throttle, but fine when putting around town)