Three lights wired together on one loop connect in series with each other. In a three light loop, the electricity flows from the power source through the first light, then through the second light, and finally through the last light before returning to the power source. If one light bulb burns out in a series circuit loop, all the light bulbs lose power and turn off; if one light bulb burns out in a parallel circuit, the other lights maintain power and remain on.
Turn off the three-light loop's power supply. If connecting the light's loop to a battery, disconnect the battery terminals. If connecting the loop to a switched high-voltage circuit, turn off the appropriate circuit breaker.
Run a 2-strand wire set from the power source to the first light, from the first light to the second light, and from the second light to the third light. Cut the wire sets to size with wire cutters. A 2-strand wire set contains two insulated wires.
Strip 1/2 inch of the insulation from the end of each wire strand with wire strippers. The wire strands often have colour-coded insulation.
Twist the stripped end of one wire strand to its matching wire strand -- the wire strand with the same colour insulation -- at the first light fixture, then cover the wire ends with a wire nut. Connect the remaining wire from the power source to one of the light fixture's wire terminals, then connect the wire feeding power to the second light to the light fixture's second wire terminal. Consider the wires connected to the light fixture "Positive" and the wires twisted together "Negative."
Spin the two negative wires at the second light fixture together, then cover the wire ends with a wire nut. Connect the positive wire from the first light fixture to one of the second light fixture's wire terminals. Attach the end of the remaining positive wire to the second light fixture's wire terminal.
Secure the end of the positive wire from the second light to one of the third light fixture's wire terminals. Connect the end of the negative wire to the third light fixture's remaining wire terminal.
Connect the wires at the power source. If the three-light loop uses a battery as a power source, connect the negative wire to the battery's negative terminal, then connect the positive wire to the battery's positive terminal. If the three-light loop connects to a switched high-voltage power source, connect the positive wire to the power source's switch terminal and the negative wire to the power source's "Neutral" connection.