What could happen if you turn it on? Either it heats the water with solar or it doesn't. IF it doesn't then there is either a pump or control problem. I am surprised that the solar water heater does not also have an electrical supply to it, since it should also have been able to be used in a single tank configuration which would have needed it for backup. The arrangement of the three tanks is subject to the installers concept of which way would save the most energy. Normally, the solar tank last would use the most electricity, and the solar tank first would use the least.