You could always have an xp wheel like the mana wheel, split thusly:
Where each hero gets 1/3 of the normal experience for skills, but gets little incremental boosts to stats and buffs troops. It'd also make a hero that's maxed his skill tree useful as a general leading troops. Or allow you to focus on using just 1 or 2 skills and pumping xp into the hero's stats. Or keeping him in the back to act as a general, armed with a tiny bow and using scrolls instead of fighting.
Also it'd give you an additional choice for each hero. And make the wheel mechanic more integral to the game.