Have the hardened skin hero ability just be a dummy ability with just icon and tooltip, then have something like
When hero learns ability
if ability level == 3
remove heroHardenedSkin2 from hero
add herohHardenedSkin3 to hero
with a heroHardenedSkin ability for each level.
Might be a very roundabout solution but... eh, if it works, it works. Someone else is welcome to offer an easier solution.