1- You have 12 bbs and weight them, 6 in each side, you'll end up with 6, you used the scale once.
2- Weight 6, with 3 each side, you end up with 3, used the scale twice.
3- Weight only 2 of them. If both have the same weight, the one left is the heavier one. If the scale changes direction, you'll logically know the one heavier.