Just remember that this isn't World of Tanks. Adding an M3 isn't about "balance" or what best matches up against the Tiger or Tiger II. Its about adding a vehicle that would help flesh out the ground vehicle set, see use in the Main Arenas, and also add to the equipment available for scenarios, FSO, or other events. Furthermore, historically, Aces High has only represented aircraft and vehicles that had a significant role in the war, were deployed in squadron strength (more or less--i.e. no prototypes), and saw combat. So, don't expect a T-29, unless HTC changes their mind.
From an Aces High perspective, an M3 would add a lot more value to the game than an M26.