Although three wires from the mic might indicate a stereo mic as you suggested, it also might indicate a mic using what's called a "Balanced" connection. It's common in professional audio: A positive and negative wire for the mic condensor as well as a third ground wire in addition to the shielding sleeve.

Also, some mics do cheap noise-cancelling by having a second mic facing away from the mouth of the user, and combining that inverted signal with the main mic. It could be that, too.

So, lots of possibilities. Good luck.

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Tony Fabris