I need help with IMP 1 pow 15?

A king has all the gold in his kingdom in twelve bags, with each bag holding the same amount of gold. He finds out that one of the bags has counterfit gold in it, though. He doesn't know whether the fake gold is lighter or heavier than real gold, and he only has a pan balance scale. He has to find a sure way to discover the fake bag in only three weighings. You can weight more than one bag at once against each other.