I prefer to use a calorimetric flow switch. Try weber Sensors Inc www.webersensorsinc.com and check out the model 4100 coolGuard and 4120 metering flow switches.
One method would be to monitor total flow of water to the fields. If flow exceeds the amount for three fields, the flow monitor would alarm and/or shut-down the pump.
Calorimetric flow sensors or "flow-captors" are ideal for this application as they have no moving parts and are...