Take 1 coin from the first bag, 2 coins from bag 2, and 3 coins from bag 3.
Weigh them. If the weight is 6 pounds one ounce, you know the first bag has the fake coins because there is only one fake coin in the pile. If the weight is 6 pounds 2 ounces, you know the second bag has fake coins. 6 pounds 3 ounces means the third bag has the fake gold