Alternating Lead/Lag Pumps

Hello, 

I am modeling an existing water system in WaterGEMS and I am having trouble implementing the scenario that is used in the existing system. Currently they have 3 pumps in a Lead/Lag system that turn on based on the HGL of the water tower. One pump directly fills up the water tower, and the two other pumps fill up two hydropneumatic tanks that are directly connected to the system. The pump that directly feeds the water tower is initially labeled the "Lead Pump" and the two other pumps are "Lag Pump 1" & "Lag Pump 2". Once each pump is cycled on and then off, then one of the pumps that feed a hydropneumatic tank becomes labeled "Lead Pump" ( it was previously labeled "Lag Pump 1") and the other is "Lag Pump 1" and the pump that feeds the water tower becomes "Lag Pump 2". The system alternates Lead & Lag Pumps after each is cycled. So, my issue is how do I set up my controls to model this?

Thanks,