I currently got a miltek cat back on there so if I get a 3" down pipe that will be ok?
Would I also benefit from a forge tip? If so where can I get one? Thanks 
If it is a true 3" downpipe then you will need a reducer.
If it is a 3" downpipe which actually reduces at the end (which some do) then it should be ok.
An aftermarket TIP would also be beneficial, forge are good, I have a Venair one which is excellent. Some people also run Creation Motorsports ones which can be had cheaper.