Quick Answer:Their weights are:

 = 2 pounds = 3 pounds = 4 pounds

Complete Solution:

There are many ways to solve this problem.

 Stategy #1: Add the weights on all 3 scales, so, + + = 18 pounds This means that: + + = 9 pounds But from Scale #2 we know that: +  = 7 pounds So subtract and find: = 2 pounds Since the little fish weighs 2 pounds, and we know from Scale #3 that: +  = 5 pounds then, = 3 pounds Also, from Scale #1, +  =  6 pounds So, = 4 pounds
 Stategy #2: If you put the fish with weights 6 and 7 together on the scales, then + + = 13 pounds But we know from Scale #3, +  = 5 pounds Subtract and find, +  =  8 pounds So, = 4 pounds

Strategy #3
Consider the weights in the two balloons showing 6 and 7. The weights differ by one pound and the big fish is on both scales, so the medium fish weighs 1 pound more than the little fish. The third scale shows that the combined weights of the little fish and medium fish is 5 pounds. Since the medium fish weighs 1 pound more than the little fish, the little fish must weigh 2 pounds and the medium fish 3 pounds.

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