You don't actually target a hero when you use their resources, otherwise Beorn would be incapable of using his resources at all.
The difference in who can/can't affected by each of those cards is in the nuance of who they target an who their action effects affect.
Quick Strike targets a character, and allows that character to make an attack outside of the normal combat phase. It targets a character, so Beorn can't be affected by it. The card effects of Quick never actually targets the enemy, just the character who is declaring a attack. Since the Smaug can be hurt by attack made by characters, he will take damage if your character is strong enough to break though his defense.
Like Angus said, Stand together is basically the same. The card effect only affect characters, not the enemy.
Thicket of Spears is the only card of the three who's written effects affect enemy cards, so Smaug would be immune to it.
This next comment is going back to some earlier posts about Beorn's immunity to attachments. There is one benefit to it. He is also immune to encounter attachments. Sack cards (Carrock and We Must Away) and Tentacle attachments (Watcher) don't affect him either.