Purchase a splitter adapter that takes your mic output and splits it into left and right channel. If your mic is mono, it will just send the same signal down both.
Like Winston Chow mentioned, you could use soundflower to re-route your audio but if that's too much hassle, the hardware solution may be a much better, simpler solution.
Pretty much any video editing software can do this - if you have Windows, you can use Moviemaker (free, bundled with Microsoft Live these days)
The idea is you populate the timeline with all your video clips, and add whatever transitions you need, then add your audio to the sound track.
The only problem is that Moviemaker does not support 3Gp, so use a ...