A four-door 3 Series is out of the question, because all M3's are two-door. So what we are talking about are the 2-door 3 Series compared to the mighty M3.
Multi-reflector on 3 Series. Xenon's on M3.
M3 alloys set them apart. Wide M3 wheel arches extend almost till the bonnet.
Bumpers are different.
3 Series has twin exhausts. M3 has quad exhausts.
Side grille behind front wheel arch not present on normal 3 Series. Only M3's have them.
Also notice that the side indicator light is on the side strip for the M3, while its above the side strip in the normal 3 Series.