M3s power comes from forcing exposed single units to retreat and running them down. when you see M3s immediately get to cover and sit on it. start concentrating your force from there. If you get an mg42 (which I strongly suggest) always have a gren squad on top of it to faust the M3 if it comes by. The key with dealing with M3s is to not allow them to pick off exposed units. They hardly win vs units in cover unless they are guards in an M3 or just simply more of them.
A short summary of what to do is Concentrate arms. It is a strategy that you have to adapt to when you see it otherwise it will pick you apart.
I know how to play. It's not a L2P issue. There is this weird russian spam player. He does the same shit EVERY game and i just cant beat it (Cpt.Molo) His micro sucks but since loosing 1-3 units doesn't matter with this strat because germans will be dead at the 5 minute mark he still beats me everytime.