Those shifters won’t work, the travel is far out, you need to use a stock old style trigger and add a middle gear!
It seems that Clever Cycles have made it work:
https://clevercycles.com/blog/2017/03/03/want-a-9-speed-brompton-the-ti-parts-workshop-triple-cog-kit/
Interesting! I’ll look into it further...
@Brommers started
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Those shifters won’t work, the travel is far out, you need to use a stock old style trigger and add a middle gear!