Some incentive for side balance is already in the game, based on perks. It has limited effect. So, what are some other ways?
I think the best way would be for an adaptive system (basically a control system -- like a thermostat for your furnace) that adds incentive until the imbalance goes away.
You could do the following every 15 minutes. If a side has 10% more players than the middle-player side, add five perk points to all its perk aircraft and add 5 perk points to its two lowest-ENY non-perk aircraft (making them perk). If a side has 10% fewer players than the middle-player side, subtract 5 perk points from all its perk aircraft (eventually making some of them non-perk).
It's like a thermostat in that, as long as there is imbalance above a threshold, changes are pumped into the system. Once there is no imbalance, no changes are made. If there are imbalances over threshold the other way, changes are done in the other direction.
It is a self-balancing system. It is based on perks, which is already a part of the game (so no radical rethinking is needed on players' parts). It enhances the use of perks and ENY. It will help with diversifying the planes people fly. It might be easier to put it into the Aces High system as the system already has code in it that adjusts perks periodically.