3" damn that's pretty big...ima go 2 1/4 or maybe 2 1/2 to a borla muffler...I had flowmasters and those are too high pitch for me...borla has a lower tone to it which I like...
ok so i take a old muffler and gut it out, a straight throu muffler has a baffle tube in it i make a wrap of heavy duty steel wool that surrounds the baffle tube i go for 1 inch of coverage. then i wrap it in muffler packing a fiberglass Kevlar bi-weave then i get a nice stainless tube about 3inch diameter and stuff the baffle tube in the muffle sometimes it need a sledge to get it in packed good. weld on some caps for both ends with my inlet pipe and outlet pipe and bam homemade muffler straight through flow muffled noise little to know power loss like the chamber mufflers give you. mine tend to be a bit louder but if thats the concern you can pack the center of the baffle tube with some heavy steel wool to cut down the noise.
heres a link How to: Make you're own muffler. - RX7Club.com