They have one outlet...sometimes. Sometimes they have two.
Well now, that's clear as mud.
Ok, the porting (inlets and outlets in laymen's terms) is determined by what type of three way valve it is; either a mixing valve, or a diverting valve.
Mixing valves have two inlets and one outlet. Diverting valves have one inlet, and two outlets.
The common port is normally marked AB, and the other two ports are marked A and B.
Either valve can be used to vary either flow or temperature, depending on how its piped.
As with a two way valve, the three way valve generally shuts off against the direction of flow to stop water hammer and allow smoother control.
And I almost forgot; they're either open, closed, or somewhere in between.
Next article we'll talk about how to pipe them.