Same as the other one? I just changed a little the procedures for this one.
Take 7 coins, 1 from the first, 2 from the second and 4 from the third.
7 coins make 70 ounces.
So, if you weigh 72 ounces, the bags containing the 12 ounce coins are 1st and 4th.
74 -> 2nd and 4th
76 -> 1st and 2nd
78 -> 3rd and 4th
80 -> 1st and 3rd
82 -> 2nd and 3rd