Water irrigation system - control three valves from one pin, how?

You could definitely do that. Try Google to see if someone else has done it and if there is a module you can buy. That kind of multi-level control is very common in the steering-wheel controls on any car that has buttons in the wheel.

But it is highly likely that you won't find a module and the easiest way to build it is to use another Arduino. I suggest the Micro is easier to work with.