Citizen folding bike 3 speed - pedal slipping
Does the chain make a ratcheting sound when you "slip" or is the sound coming from the hub?

My best guess if it isn't the chain is that either the ratcheting pawls in the hub are worn or some of the planetary gears in the internally geared hub are worn or maladjusted. 

The first thing you should do is check chain tension. If that's good, then I'd take it to your bike shop. They can quickly:


Diagnose if the shifter is shifting properly and make sure you're not between gears when it's slipping. 
Make sure the rear freewheel is working properly. 
Check the internal geared hub. 


The first problem is super cheap - a simple adjustment. The second and third are more expensive, especially if the hub needs an overhaul or replacement.