A parallelogram with three congruent inscribed circles is painted across a 20-foot by 12-foot rectangular banner as shown. The longer sides of the parallelogram have length 15 feet. What is the radius of each circle? Express your answer as a common fraction.
I thought the three diameters of the circle added together would result in the 12-foot height of the circle, meaning the radius of each circle is 2, but I think there is more to it.