How to test for live wire in a car using an analog multimeter?
Don't try measuring current and don't allow any of the three wires to short or you'll blow a fuse or melt a wire...

Don't try measuring ohms. Set the meter to read DC volts (not AC)

Connect one lead of your multimeter to the battery ground terminal (presumably the negative terminal) and turn your lights on as if to activate the normal beam (low).

One of the three wires should read +12V dc. Check that the other ones don't because if they do then you may have to try a different approach.

Hopefully one wire is registering +12V. Next put lightswitch onto full beam and check which one of the three wires registers positive 12V - it shouldn't be the same as the wire that went positive on the low beam test.