Nice diagram. I see 2 potential problems that might not really be problems but you should think about.

First, the pressure drop across the right set of panels is going to be twice what it is across the left panel. Not sure what the result might be, but "smooth flow" through a solar system makes it more efficient.

Second, the water entering the 2nd panel on the left (in series) is already heated, my guess is that the 2nd panel is not going to be as efficient. Any type of heater works best when the tempature differential is as great as possible.

Seems like it would add only a little to split your input into 3 streams to feed each panel, then join all three on the return side. I think it would balance the system a lot better and the pressure differential across all 3 panels would be the same.