Over the course of the past few prototypes I tried a lot of versions of the gearing.

For instance in this mess above you can see a front derailleur mounted so as to switch between the three gears on the pedal chainring. All the solutions were heavy and messy. 

I decided to worry about the gearing later and focus on a less expensive, lighter and simpler solution. I realized that with jockey wheels, commonly used on rear derailleur's, I could route a simple loop of chain around all the required sprockets.