how to shorten christmas lights with 3 wires

The first step is to identify which wires are live and neutral. You can do this by using a voltage detector to find out which wire has the current flowing through it. Once you have identified the live wire, cut it and strip off the insulation at one end. The next step is to attach the live wire to one of the ends of a resistor. The other end of the resistor should be attached to the neutral wire. This will create a short circuit between the two wires, which will effectively turn off the lights.