I have the xforce header back system and I don't know if they make 3 inch adapters. On my system the whole kit uses flanges from the headers, to the cats and lead pipes all bolt together. I think if your trying to connect the xforce catback to different brand headers you will have to take it to the exhaust shop for 2 custom pipes.
I don't have any of under the car but American muscle has pics of the complete system. I didn't pay their price for it though. I paid $1300 from summit on a closeout deal.