Most switches will require three wires because you are throwing a switch in two different directions. You therefore need two circuits. One wire, the common leg, is used in both circuits.
Each of the other legs complete the circuits for one direction or the other. The current cannot be on all the time, so you control the turnout by giving the switch machine a burst of energy via a MOM switch. This MOM switch is on the individual leg, not the common.
Sometimes, but not always, the MOM switch that controls the current, controls the turnout in both directions by pushing it opposite directions. In this case, both of the swtch legs are wired to the same MOM.
This sounds confusing because the momentary switch, the MOM, is called a switch and the swith machine is called a switch. Good thing model railroaders call the track switch a turnout or we'd be going nuts about now.
