Answer:
For gallons per minute, you would do 36 divided by 3, which is 12, and 3 divided by 3, which is one, so 12 gallons per minute.
For minute per gallon, you would divide 3/36 and 36/36. You could use a calculator.
You do this because you want to find the per, which is one or single, so you would have to divide by the number you want the per to be in.