unless you're having custom DP's made that do aren't currently available comercially, then you will actually have 3" downpipes that taper to 2.5" and then I guess your exhaust may open back up to 3". Not a true 3.0" exhaust unless you've already made plans to do what I've done here:
http://www.e90post.com/forums/showthread.php?t=367906
And instead of tapering to 2.5" after the mids, continue on with 3.0".