Can you synchronize them by time and have the ME3 run a dummy solenoid when any of the other controller zones are running? The ME3 will think the flow is its dummy zone. Set the flow sensor alarms and such off for the dummy zone or set the flow rate acceptance bands wide enough so it won’t alarm. Just a thought that hit me as maybe would work. I’m pretty sure the ME3 can keep very accurate time — what is the other controller? — if it’s clock is accurate enough, synchronization might well work.