external 3 speed?
Yeah, the Bikegang kit basically
https://www.bikegang.co.uk/bikeganghut/Brompton-2-to-3-Speed-Sprocket-Cog-Thumb-Shifter-Kit-MiniMODs-x-Sturmey-Archer-p115439619
"best" is a sunrace M90 friction
The Bikegang photographs on the website you linked to show it being used with the Sturmey Archer SLS-30 shifter, which looks like it would provide indexing for the middle sprocket.
@neu started
London Fixed Gear and Single-Speed is a community of predominantly fixed gear and single-speed cyclists in and around London, UK.
This site is supported almost exclusively by donations. Please consider donating a small amount regularly.
external 3 speed?