Strida single speed - upgrade to 2-3 speeds possible?
3-speed is possible, but not at bottom bracket without heavy modification to the frame. The usual location for 3-speed gearbox is the rear hub. As far as I can tell, one could rebuild the rear wheel with suitable 3-speed hub. Running the shifter cable to the hub might be complicated with the folding frame, but I don't see how it would be more difficult than running it to the bottom bracket. Some models already have disc brakes, which also suggests that you can run a cable to the rear hub. If disc brake is not required, another option would be kick shift 2-speed hub. These things have a coaster brake and shift gears every time you brake, no cables required.