Looked at adding a Schlumpf to my old 1x1,but stopped cold when I saw the price tag.

FSA makes one,but I can only think of one bike that uses it(Raleigh I2x8).

Truvativ also makes one,but it's designed for MTB use,is as expensive as the Schlumpf,and if memory serves you need a special mount on the frame.

Strida makes a 3spd,but only for their folders.

I've looked at them,and think they're pretty much a niche item. If they could make them fit in just the BB,so you could use any cranks,and could do something about the jumps in gearing(the Strida EVO 3spd is a better idea),they'd be much more practical.