Interesting idea, but where you're going to have problems is getting something like a rear der or a chain tensioner on there -- it doesn't have a rear der hanger to mount it to. I'm surprised you can even do a 3 speed, what are you using for a cable stop? I guess you could bolt something on the chainstay or some such.

I'd stick with just a regular 3 speed, you actually get a pretty good range out of them. You can swap out the cog to change the overall ratio.