Wiring a DC socket - why three pins?
This might be something similar to the female single channel audio connector where two of the 3 pins will be shorted when unplugged and when its plugged one of those 2 ping will be floating which usually goes to a battery and the remaining 2 pins will be connected to your plug.

This diagram might help you understand better.



Here pin 3 will got to  the battery, pin 2 &amp; 1 to the circuit.

When you plug in the socket pin 1 &amp; 2 would provide the power to the circuit and battery will be disconnected.

when unplugged the battery would be connected back to the circuit.

Though there are other configuration also in which it can be used but this is a simple example which might help you understand the way these socket works.