2_i 

Thanks! Brompton actually has a rear derailleur. It is quite modest. Basically there are tweezers under the chainstay that move the upper pulley left and right, bringing it under the left or right cog, depending on the pull in the shift cable. The functionality is like that of a typical rear derailleur - it just does not quite look like the typical one. In the modification you make the tweezers rest in one of the three rather than two positions, after adding the third cog. You set the pull of the shift cable with a standard 3-speed indexed shifter. For 3 positions you do not need about the amount of pull in the cable as the extreme positions of the derailleur are set with limit screws.

This will have to wait as I am off to work travel in the morning. There are overlaps, but it is good too. I think that numerically my gears are better than in my main bike. The latter is justified as my vicinity is relatively flat and the folder may be taken to quite extreme places. However, operating these 3 shifters can be confusing. I got it under control with 2 cogs, but with 3 there will be some learning curve.

Wow! This is impressive. I'll be biting my fingers concerning the Dahon, but it would be good to learn details on the Cannondale in the meantime. I had 36 speeds on my Trek, but then went down to 32.