Some people over
www.torontomazda3.ca have got their rust problems fixed under the new, extended warranty, so I'd recommend you to search on the subject on that forum. I think by now the M3's rust issues is well known by both Mazda Canada and Mazda dealers, and If you can reach to the right person/dealership you might get it fixed under warranty as well.
However, even if you get lucky and get it fixed under warranty, I'd suggest you to sell it before the rust comes back, which is usually much shorter than expected in M3's case.
BTW there was a mazda rep posting on this forum not too long ago as well, you might want to contact him too, afaik he/she was talking about the subject as well..
Good luck