Microphones ... how to get stereo recording from one mic.?

Here's my setup: I'm running two "lines in" from a digital keyboard into an interface linked to my computer. I want to run a mic (3-prong) into ONE of the two possible mic receptacles of the interface so I can add voice to music from the keyboard and have them both recorded simultaneously.

Here's my question: I know the music from two "lines in" comes through as stereo, but will the mic, because it is using only one receptacle, come through as stereo also, or will it record as mono?

I would like some direction on this before I go ahead and purchase the mic.