Delta_G:
Is there some way that one could control the three valves from one pin on the arduino?
sure, if you want all three valves to do the same thing.
Actually, there are several ways to control multiple things with a single wire but it takes more circuitry and potentially some clever s/w.
There is the standardized Modbus interface, but those devices are expensive.
It could be something custom like using analog voltages created from PWM signals, frequencies that are filtered using notch filters, or a simple serial (not async) interface that can be decoded using an RC network and a shift register.
I have used the latter to control a hd44780 LCD from an ATTiny part.
But all those are much more complex than just using multiple Arduino pins.
--- bill