I’m not against giving them Lava Resistance, but without Lava in the set it seems a bit of a waste here.
As far as a special attack version of the drop goes, that could be extremely potent as an attack, especially if it can be done early on. Maybe not as devastating as the Airborne Elite, but it can be more. Bombing the enemy Startzone before they even have a chance to react is really strong. Like, really really strong. Placing one figure at a time would be less dangerous but also less appealing I’d think.
A simpler idea I had was to just one of them ressurect adjacent to each other each turn via...
Rise from Ashes
At the beginning of their turn, you may place a previously destroyed Phoenix adjacent to another Phoenix you control.
Then just give them a statline based upon how much you want them to cost.
This version also allows ressurection synergy with other Phoenixes, without letting you bring them back after all 3 are dead.