I like that what you suggested puts all 3 radiators in parallel, and that none of them overlap.
I think that testing that without an additional pump, and then with (with the second pump in true parallel and equally matched) is a very good idea to properly isolate if flow/pressure really are the problem or not, or if air flow is the problem. (aka I think you designed a good test setup).
I also like in your drawing that it seems only one radiator has 10 AN and the other two now get 38mm. If in your custom radiator you can increase the size to something larger like you suggested, like 14 AN, I would.
I know its a lot of work, but doing things the right way usually is, lol
I think that testing that without an additional pump, and then with (with the second pump in true parallel and equally matched) is a very good idea to properly isolate if flow/pressure really are the problem or not, or if air flow is the problem. (aka I think you designed a good test setup).
I also like in your drawing that it seems only one radiator has 10 AN and the other two now get 38mm. If in your custom radiator you can increase the size to something larger like you suggested, like 14 AN, I would.
I know its a lot of work, but doing things the right way usually is, lol
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