A 3-way valve serves 3 functions. When the knob is oriented perpendicular to the direction of the arms the two sides are isolated and the valve is in closed. When the knob is orientated in the same direction as the arms then one of two pathways will be open.
When it is 180° from this orientation but still in the same direction as the arms then the other pathway is opened. Ideal for connecting vacuum pumps to a distillation system to isolate the pump, pull vacuum, and release vacuum.