bperrybap:
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
Actually...
I went on to say in that post that you could conceivably create this with three relays, a uc, and some way to communicate from your first Arduino to the one with the relays. But at that point, why not just run them from the first Arduino.
And it becomes a question of why do you want to do this. If there is a real problem then there may be a real solution. But this is an example of the x-y problem as it is.