It doesn't have to be 3 separate minis; in fact that would be somewhat wasteful and expensive. Take advantage of the fact that airmech mode is in between battlemech mode and fighter mode. Just do the fighter and battlemech minis and use them to build the airmech.
Sculpt based on sections: battlemech torso, airmech torso/upper fuselage (the bottom being sculpted to fit the limbs), arms, legs(actually everything below the waist) and the lower fuselage (the bits that the limbs make when folded up, egines, guns, etc. sculpted to fit over the parts limbs would attach to in airmech mode). Attach the limbs to the battlemech torso to make the battlemech, attach the limbs to the airmech torso/upper fuselage to make the airmech, and attach the lower fuselage to the upper fuselage to make the fighter. As long as hey sculpt everything to fit together, they can just make minis for two modes and get the intermediate mode without having to spend the money to sculpt it separately. Put all the pieces in the same package and let us assemble either an airmech or a battlemech and a fighter or buy two packs and get all three modes.
I wouldn't be adverse to a smaller scale fighter mode mini as well.