A three wire valve is a divertor valve and is only open to one port at any time. A five wire valve has a midposition so you it will open to two ports at the same time if required to do so.
Three wire= Live, Neutral and earth........puerly to wind a motor
Five wires= Neutral, CH on, Hotwater off, Permanant live and switched live.
The Chentral heaing on wire will drive the valve half way to the mod position but when the c/h on and DHW off are live it will drive the motor all the way to the open one port only.