How to quiet down my 3" exhaust?

A second set of turbo mufflers as long as you can stand would be my choice. For that matter the magnaflow style should help as well.

Not a direct comparison but my D150 was obnoxious loud with headers, flowmaster y-pipe and a 3" single exhaust with a summit racing 3" in/out straight through muffler. Unbearably loud at WOT . I put a 3" Dynomax super turbo in line just after the straight through muffler and before it goes over the axle and it is night and day difference. Arguably the best sounding exhaust I have run on anything.