When you said one looped system with three tanks and a sump, I think that means you have water from the return pump go to one tank, then drain to another, then to another, then back to the sump.
That necessitates having each tank in the water path at a lower height than the next because you will be doing a gravity drain that way.
But when most folks have multiple tanks plumbed into one sump they do it another way. I have two tanks serviced by one sump, and each is plumbed independently. In other words, each has its own return and drain line. I would recommend you do this if possible. You can tee the drain lines from two tanks together as long as one line can handle the flow from two tanks to the sump, but for water return, what I would do is use one pump and make a PVC manifold to distribute the water to each tank's return line. You use PVC ball valves on the return lines from the manifolds to control the flow to each tank.
