1.divide the 12 balls into 2 groups (6 a group). Weigh the first time. discard the lighter group. 2. divide the 6 into 2 groups (3 a group), weigh the second time. discard the lighter group.
3. pick up any two balls, weigh the 3rd time. If they are equal, the one not picked up is A. If not, the heavier one is A.