Heroic and iron give you 1 additional star each, giving you a maximum of 5 per map (3 stars for normal).
Thing is, I never really got to apply those extra stars usefully. Early iron and heroic maps limit your upgrade level. That is, you may have enough stars to max your catapult and wizard bonuses, etc., but the map will ignore any levels above the set limit. So if you've beaten the normal game, you have enough normal stars so that the heroic and iron stars aren't useful early on. Perhaps they become useful mid-game or later.
On the other hand, if you're beating the iron and heroic levels as you progress through the normal game, those bonus stars will be useful for the normal maps. However, if you're beating the heroic levels, the normal levels are comparatively cake, and those extra stars, though applicable, won't matter much to you.
Heroic is flippin' hard, though. I didn't get very far. That said, I'm not really adept.