IF its mostly stock .. I would just leave the stock one on .. a 3" won't give you anything until your in the 11's
You will actually gain more from a ported elbow then you would the 3" downpipe ... The ported elbows are about $75 .. way more bang for your buck on a stocker.
This suggestion is if you actually want it run out a bit harder .. if you want to change up the downpipe just because then It should be pretty easy to come up with one ..
On another note I assume its chipped and has all the little work done.. as there are noticeable gains there.