how to quiet down 3 inch exhaust

As mentioned above, cut a foot out of your tail pipes & put something like this in there. Or a longer one if you have the room.

http://www.summitracing.com/parts/JEX-A4312B/

Years ago JC Whitney used to sell something called the Racket Buster that was pretty much something like this but, IIRC, with a chrome tip. A lot like what was suggested by 340 8BBL.