I cant see HTC re-modeling the M3/M16 so they can add the M3A1. the M3 is of the same category as remodeling the the "troops" that head into the map room. The M3 might look nicer, but a remodel I doubt would change its speed, turn radius, gear ratio, cargo capacity, etc.
Again, Im not a coader, but one would think that adding the M1A1 gun to the rear of the M3 would be about as easy as it gets for adding in new vehicles into the AH game. Same goes for the SdKfz 251/22 by simply adding in the Pak 40 75mm AT gun (same as Pzr IV).