S = length of shortest side
"the length of the longest side to be three more than twice as long as the length of the shortest side"
The length of the longest side is: 2S + 3
The length of the third side is 12 ft.
perimeter = sum of the lengths of the three sides
perimeter = S + 2S + 3 + 12
perimeter = 3S + 15
Answer: 3S + 15