A 3 pin XLR allows for a balanced signal path and also allows for 48 volt phantom power which all professional microphones require. The pin layout is: Pin 1/ground, Pin 2/positive signal, Pin 3/negative signal. Having a balanced feed helps to reduce/eliminate grounding issues and RF noise problems. Stereo & surround sound is created in post production. Dialog is always mono as it works better with subject placement in a shot.
Eric