If you already have BWR and put 3 cogs on you get 9 speeds, not much different from 10, and the upgrade cost is minimal unlike with the 5-speed hub.
asy
it doesnt work like that friend.
3 cogs in bwr is a troublesome mod with many details to be adressed.
here , its just a glimpse how it is done:
https://www.clevercycles.com/blogs/news-and-events/want-a-9-speed-brompton-the-ti-parts-workshop-trip/