I had a Shimano Exage400 LX triple crank from another bike that I put on yesterday. It fit perfectly and the chainlink was good so now I have an 18 speed folding bike out of my Single speed!
(2 cogs on the rear, 3 internal and triple crank on the front)

I will do some calculations to figure the best combinations of two rear cogs, internal ratios and triple front chain rings.
Found a 34.9 braze on adapter that will fit the main seat tube for the front derailleur, but might just use the greasy finger method for now. I rode the bike with the chain on all three chainwheels and it was nice and quiet, so now just need to figure out the shifting hookups and I am all set! Will post updates as I progress.

On calculating the inch gear, do I use 20" or 19" for exact gear ratio? I measured the tire diameter and it is 19 inches diameter, kenda 1.5 on 406 rim.