I'm not sure how you define a good company for this warranty, but my guess is that there are no options out there. Not speaking to the company but how much you will have to pay for an extended warranty on an M3. Like many insurance policies,( and that's what an extended warranty is ) these warranty policies are priced, in part, based on the losses paid compared to the premium earned. The E46 M3 reliability is so pitiful that the pricing of those policies must be huge. Before we have to discuss reliability of these cars, please refer to all the complaints on this forum, and others. I'm basing my relaibility comment only on what I've read here, and elsewhere. My car had all the factory issues cleaned up before I bought it, and I've had no breaks, etc. so far.
However, when this car breaks, most of you may know that it costs a lot to fix it. The warranty companies know what breaks, how frequently the breaks occur, and how much it costs to fix those breaks.
Suggest you contact those 'stealers" everyone seems to hate so much. Each one probably sells policies from more than one company. See if you can get a price for your car, then decide what's right for you.
FWIW, when I bought my E 46 M3 2 years ago from the 2nd owner, the best price I could find in the Northwest area was about 3X the same policy for a comparable Asian car.
The newer M3 models may have improved so much that such policies are readily available and / or relatively inexpensive. Let us know what you find in the marketplace. Good luck.