Did you forget to mention that real gold coins weigh 10 ounces and fake gold coins weigh 9 ounces? (or am I thinking of the wrong puzzle?)
Anyway, you take 1 coin from the first bag, 2 from the second bag and 3 from the third bag. Take all the coins you removed and weigh them in one go.
First bag is real if weight is 55 (=1(10) + 2(9) + 3(9)) ounces
Second bag is real if weight is 56 (=1(9) + 2(10) + 3(9)) ounces
Third bag is real if weight is 57 (=1(9) + 2(9) + 3(10)) ounces
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