To turn it into a 3 speed you would need a new rear wheel with a 3 speed hub. These are hard to find cheaply. You could try looking for another bike that has one and swap wheels (plus cable and shifter). Cheapest way to add gears I found is to go for a deraileur. Just buy a cheap kids mountain bike with 20" wheels and you have everything you need. Whereabouts in the world are you Duo?