Jimmy weighs 75+6 = <<75+6=81>>81 pounds.
Adam's weight is 75-15= <<75-15=60>>60 pounds.
In total, the three weigh 75 + 81 + 60 = <<75+81+60=216>>216 pounds.
The average weight of the three is 216/3 = <<216/3=72>>72 pounds
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