How to quiet down my 3" exhaust?

Frankly Doc, what you have now is what i would have recommended to someone else as a way to quiet down a too-loud system. The turbos are quieter than same-size chambered (flowmaster or clones) or straight-thru (ultra flow or glasspacks). I dont think a second set that far back will have much more effect than the glasspacks/resonators you are using now
I would think that your best bet would be a large quieter muffler, triple pass, in your stock location. Maybe a hemi muffler, or one of the big walker/dynomax/magnaflow versions.
I am using a pair of 50series fm big block on my 62, 5x12x22" case, also with a fuel cell in the trunk. But if yours is too loud with turbos, i dont think they are quiet enough for you. I also have a 3" 70 series fm big block that is very quiet, but it is in a single exhaust system (dually pickup with a cat). I also have a looong case magnaflow 3" that is fairly quiet for a staight thru.
The secret (at least in my humble opinion) to a great flowing quiet system is..... large tubing size, and the longest biggest case muffler you can fit under the car.