In many cases the m3 problem is magnified by the spawn point placed too closer to the town. The drive distance is so short that the m3 can be killed again and again, but it will keep showing up every 2 minutes. An attacking plane that runs out of ammo, fuel, or gets shot down will take much longer time to return.
The need to get the supplies just to the edge of town has a similar effect to the above. If the M3 had to drop the supplies at the flag, it would make its travel time longer and give attacking tanks a chance to intecept it, or prevent the resupply by holding the flag. Currently, the m3 can come out of the trees line to the other edge of town and make the drop before planes or tanks can intecept it.
1) Make sure that spawn points are at least a few miles from the town - travel time should be the same as a round trip time of a plane from the near air base.
2) The m3 town resupply should only take if it is delivered to the flag. C47 town resupply can have a greater range (and larger effect) than the M3.