I would not consider the M3 a supercar because it falls a bit short on power and exclusivity. I also wouldn't call any 911 a supercar because while some have the performance, like the m3, the body style is all over the place.
Lambos, GT-Rs, Ferraris, Pagani, Koennigsegg are what I would call supercars.
The M3 is a lot of bang for the buck. The GT-R is the only car I think delivers more bang for the buck.