Hot water heating system problem?

Working on solving a hydronic heating problem at a friends new garage. Its a heated slab and the guys that installed the slab, also laid in the PEX tubing and did it very wrong. They were told to keep the loops less than 240' (1/2" pex) well being given three 500' rolls they laid in 3 loops 500' long. Big problem is that my buddy and I were gone that day and the slab was poured and finished when we returned late that night. Each loop will have 51' head loss at 3 GPM. Any suggestions on salvaging this mess? circ pumps in series? One big pump? trying to solve this for hundreds not thousands.