Maybe having a manifold would be better on the intake. That way you divide floe to the three pumps right away thoer wise the you are going to have to size up and down a lot.
I think I understand what you are saying, but:
All of the pipe, except that immediately touching pumps, is 1", so I don't think the sizing is going to be an issue.
Also, since this utilizes the advantages of a gravity fed system, I don't think my intake will want for lack of water.