You will probably see more of them in future maps (especially post normandy ones). I kinda prefer it to the Thompson.
I kind of prefer it too. M3 has lower firing rate, and it's easy to control recoil for fun spray sessions.
You can also find the M3 on Operation Cobra, but only as pickup kits. A few are found in the north main uncap base. You can also find one spawning at the church and Heb airfield if the US controls that flag.