Do you have the stock midpipe? If so, is the cat back a "flat" style flange? It didn't bolt up perfectly did it?
I have a similar set up, except mine is the stock catback, with a flat flange DP (read helix). Now, my leaked at first, until I got some gasket maker from autozone, and cut out about a 1/4" worth of gasket material to seal the two pipes. No leakie in 1.5 years.
To cut the gaskets it's easiest to buy a metal gasket that is made to mate a 3" exhaust at that point, use that as your template and cut away.