I know the pipe it's self is 3" when you have the bigger style but the coupler that goes on to the turbo is slightly bigger only reason why I thought 3.5". Could depend on what clamp you get tho and how much extra slack they give you!
3" pipe takes 3" clamp. Only time I used a larger one was when i had a really really stiff/thick boot connecting the pipe and I/C. Then i used a 3.25" clamp