Trying to install security lights and found a 3-wire in the attic. The bare wire and the white wire complete a circuit. What can I do?

It is possible the white wire is hot if it is part of a switch loop. Where does it connect? Where does it come from? What is the other wire-you mentioned 3.

What is the third wire?

Just exactly how do the white wire and the bare wire complete the circuit? "Bare" is the
Equipment grounding conductor and is NOT part of the circuit.

A normal circuit is made up of a hot conductor (usually black) and a Neutral conductor (usually white)

The bare conductor keeps all of the metal in the circuit at the same potential level so you do not get a shock if you touch the metal. It also functions to clear a fault or short circuit.

Just exactly how do the white wire and the bare wire complete the circuit? "Bare" is the
Equipment grounding conductor and is NOT part of the circuit.

A normal circuit is made up of a hot conductor (usually black) and a Neutral conductor (usually white)

The bare conductor keeps all of the metal in the circuit at the same potential level so you do not get a shock if you touch the metal. It also functions to clear a fault or short circuit.

If the white wire was hot from a switch loop, the ground would complete the circuit and measure 120 on a VOM if white and ground connected.

Agreed, but the OP did not provide a full description of the wiring. If the supply is going directly to the switch, then a 120 v between the supply's neutral and the bare EGC would indicate a serious problem.