If this is house wiring and it is AC Volts, I would first measure between a known good ground and each of the wires to look for voltage, Identify any hots and cap for now, then measure resistance between each of the remaining wires and ground. Resistance could be a bulb, a fan, or anything plugged into receptacle.
Always check for the pressence of voltage before doing resistance measurements.
On house wiring probes can be reversed and will not be a problem. If you were measuring Batteries or DC volts it would read a negative number on a digital meter, and on an analog meter needle would want to "peg" backwards, just reverse leads.
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