so i have a invidia s pipe and me and a friend are thinking of swapping exhaust will a 3 inch fit the invidia s pipe

Yes, depending on what your going to use for the 3 inch.
I'm assuming this is for a turbo, so I would just ditch the Invidia.

You could use it but the connector would have to be 2.5" like the s pipe.

so the invidia is 2.5 inches

I think it is actually less it is like 2.3 or something it is defiantly not 3"

i hope this is for a turbo or even SC.

invidia is less then 2.5 i think 2.25"

its 2.38" inner diameter, 2.5 OD

i am na right now installing my kit soon

ok aslong as there was a legit reason for a 3" exhaust