I also can recommend bimmerforums, but would suggest using bimmerforums or one of the other big BMW sites to search for a more specific forum for the E46 M3. For my BMW E30 318i I almost exclusively use a small forum just for the M42 engine (m42club.com) started by frustrated bimmerforums members. And I never would have found m42club.com via googling for a 318i forum.
I find the small more specific BMW forums easier to use and get good information (like the entire 968 community). The big general BMW forums just have too many people participating with too much cosmetic banter, and it seems like every BMW variant has an strong enthusiast club and forum to go with it. Although, with the M3-series this seems to be less of a problem as their forums do tend to be smaller with more knowledgeable people (on bimmerforums at least).
Glau