may be a stupid question but here goes...

I have a 3" catback currently installed, and I just got a Megan 3" test pipe from a member here. My question is, can I install the test pipe even though Im still using the stock dp or do I have to wait until I get a 3" dp?

 

You should have no problem running with the test pipe and stock dp.

 

Your fine, your still following the exhaust rule (from rear bumper to front bumper)