First question is what type engine are you running to need 3 inch exhaust? Street engines that aren't full race built don't need 3 inch exhaust. The back pressure needed for street engines can be exhausted through 3 inch pipes and it might kill performance. I have 10 second big block cars running 2.5 inch pipes out the back of the car and tried 3 inch and lost et and low end power so switched back to 2.5 inch. Just curios what your engine is?
Proper exhaust-pipe size is a function of both engine output and displacement. The higher the engines output, the larger the pipes should be. Likewise, larger engine displacements call for bigger pipes. But you dont want to become a victim of Mores Law, either: While up to a point reducing exhaust restriction generally helps both power and gas mileage, going too large can over-scavenge an engine and actually decrease engine output as well as fuel efficiency. The accompanying exhaust system size guidelines are compiled from Flowmaster Mufflers and RoadkillCustoms.com data. To avoid becoming a restriction, all muffler and catalytic converter inlet and outlet sizes should correspond to the exhaust pipe sizes, and design your exhaust system to use the minimum number of bends.