post this in the ss/ fixed forum. They could help you more then us. IMO 1 gear is perfect, don't add 3. One of the main reasons behind ss is the simplicity. In order to have more than one gear you need different lengths of chain. right? So a long chain and a tensioner is used. Just having 3 rings and a derailler up front wouldn't do it. You would need to put some tension on the chain... There are hubs that have 3 internal gears in them. Rolhoff *sp I think. Many people use those. Years ago Schiwn used to put them on many bikes. Check ebay, you can get them new and used.
Oops..see I told you I was a newb..Didnt even see the SS forum, I'll cut n paste a copy over there.
Thanks for the input..simplicity is a consideration..but I'd think I'd want to have the option to get up and/or down a gear to vary with the terrain.