As usual the original poster didn't really indicate what his intended use for the car will be. Is it a race only drag car, or a weekend cruiser, or a daily driver.

As others have stated, 3" will be very loud inside the car. And in my opinion is overkill. But if your just racing who cares how loud it is. If its a driver or a cruiser, the loud pipes can get old very quickly.

I read an article a couple years ago and i'll try to relay what it said even though I may be using the wrong scientific terms. But basically what its said was the by the time the exhaust gets through the mufflers, a large portion of the heat energy has been disapated, or maybe it was back pressure, anyway, it stated that you could do like 3" pipes from the headers to the mufflers, and then go 2.5" from the muffers out the back tailpipes and you wouldn't really be creating any extra back pressure by having the smaller tailpipes. But the car would be quieter with the smaller tailpipes.
Wish I could find the article, I'm sure its explains it better than I did.

Anyway, I would stick with 2.5" pipes if the car sees any amount of street use, just to keep the noise down.

Good luck