Three towers are three different tower buildings on three different hill. They are the hottest photo spots in San Marino. Most of the time people only take pictures for the first two, because the third tower is relatively small and you have to wall a small bumpy path to it.
Another point is that you have to stay somewhere near the first tower to take picture for the second tower, and vice versa.
This is the first tower, I took this picture from the ground floor level of the second tower.