If the enchanted creature can't attack for any reason (such as being tapped or having come under that player's control that turn), then it doesn't attack. If there's a cost associated with having it attack, the player isn't forced to pay that cost, so it doesn't have to attack in that case either.
Enchant creature
Enchanted creature gets +3/+3 and attacks each combat if able.
When enchanted creature dies, this Aura deals 3 damage to that creature's controller.
