Divide the 12 ball into 4 3ball group A,B,C and D. Measure A and B, now you know the different ball is in A,B or C,D.Say it is in A and B, A>B. , then measure A1+B2to B1+A2, if it the same, then the different ball is A3 or B3, then measure C1+C2with A3+C1,if it is the same, B3 is the different ball, if it is different, A3 is the different ball. But if A1+B2 >B1+A2, then it means A2, B2 did not change the weight difference, so the different ball is in A1 and B1, then measure A1+C3and C1+C2, you can get the different one. If A1+B2<B1+A2, means the A2 and B2 change the weight difference, so the different ball is in A2 and B2, then measure A2+C3and C1+C2, you can get the different one.