Apparently a 3-speed Sturmey isn't really very lossy provided it's run-in and oiled (92% efficient, and a derailleur setup with its two jockey wheels is probably about 95% - only a singlespeed or fixed with a clean chain makes the magic 98%). The 5-speed, on the other hand, was really bad in the lowest gear, to the extent that you might as well push on in second.