How do I test an alternator with a multimeter at the junk yard?

You need a load a supply and a way to apply a spin. I would suggest you wait until you can get it to a parts store other wise its not likely to give much in the way of correct results. Because you need to load it to its max output and you need to supply a correct spin RPM with enough torque. Remember when you apply load you will get a jerk capable of sending the alternator for a spin across the room with enough velocity to kill you if it is not tied down well. The three lines are exciter the regulator and the output and they are all normally tied to the battery. If you try to run some of them disconnected you may blow the regulator. Consult the manual for the model you are trying to test. Also don't forget the ground.