M3 is one of the slowest guns in era, with horrible fire rate, long freeze rate, and bad reload freeze. M3 users are very vulnerable if they don't aim properly.
One of the safest methods of attacking an m3 user is to wait till they are about to reload. M3 can shoot 3 times before reloading. On their 3rd shot, run towards the m3 user and attack them. They will need to reload before they can shoot. Take advantage of the enemy's huge freeze rate. M3 may be powerful, but the gun's spam ability isn't that effective.