M3 has always been one of my favorite tanks. The problem with it in the game is this: first, the main armament is in the hull, and as of yet HTC has not modeled a hull only main gun. There could be major coding involved (at least I assume there is since there are no assault guns or tank destroyers either).
Second, last I checked (granted a long time ago) when you have a "gunner" in a tank, he has control of all the guns, leaving you only driving and unable to go to any gun mount (turret or pintle). For the M3 to be really effective, something would need to be coded so that one person (assume the driver) could use the main gun while the gunner controls the turret and copula gun.
So, although I would love to see it added, don't hold your breath.