So I appreciate all info given When I said three pump in to fill I truly mean three pump just to fill. One off the thru hall, one about a foot away on intake hose, and third in the trunk. All those hooked to same hose to fill. Why would someone do that.
Its an interesting system too say the least. Or shall I say systems. Very little consistency, I think it was evolving rapidly.
I'm guessing the pumps are aerator pumps?
My SX had a pump on the thru hull and then another pump a foot from it, then into the tank. One was a fill pump (on the thru hull) and the other is the empty pump. It actually worked well, you would think running through the opposite pump would be a restriction but it wasn't much. It allowed for a single fill and drain hose and thru hull, it had a separate vent on the side of the boat.
I actually used this setup when I redesigned the system and added a bow bag.
I would check how each pump is mounted (direction of flow) and post back. depending on the length from the thru hull to the bag and the flow rate it may have needed both pumps. I upgrade the pumps to 1100 gph and hoses to 1" and it easily filled a 500lbs bag in 5-6 minutes.