Look 585, Hunter CX "Breakaway Kit", Bontrager Ti Lite MTB "69'er", Citizen Tokyo
Strumey Archer Thumb Shifters for Brompton 3 Speed
Are the Strumey Archer thumb shifters for the 3 speed an indexed shifter, friction shifter, or both?
It looks like it will work as a bar end or mounted on top of a flat bar...
I'm wondering if a Dura Ace bar end front 3 speed shifter would work on the Brompton?
The S-A thumb shifter clicks: I suppose that makes it "indexed", though S-A predates so-called indexed shifting by a few decades.
I use a SRAM Gripshift made for a MTB triple crankset as the shifter for my Brompton's S-A 3-speed hub. It works fine, though I have it mounted on the left of straight/flat bars so that the numbers on the shifter read right side up from my riding position. It does make it sort-of backwards, as "1" is high gear and "3" is low. Odds are a Shimano bar-end shifter for triple crankset would also work.
I don't know if it's still true but 3-speed S-A hubs used to be finicky about the cable tension. If you didn't set it quite right, shifting into the middle gear would put you into "neutral", where you could spin your cranks freely (in the forward direction) as much as you wanted without any drive being applied to the hub. It can be extremely disconcerting when you're riding in traffic.
I have the 9 speed Dura Ace shifters and the front is not indexed for a triple, straight friction. I don't think any version of this shifter was indexed for the front but others can correct me if I am wrong. The S.A. Looks like the best for your stated needs.