The stock Brompton 3-speed gearing is quite adequate for most riding, especially if you're only using 3-4 gears on your 27-speed MTB (you don't say which ones though). The jumps between gears are fairly big though (roughly equivalent to 2 or 3 gear jump on a typical cassette), so compared to your 27-speed MTB, you may occasionally feel like one gear is too low and the next one is too high. Nevertheless, on my a 3-speed, after replacing the rear sprocket with a 14T from the stock 13T, my Brompton has been good for climbing short and steep 9+% stretches to 20+mph "commuter racing". If you want closer spacing between gears, I heard the new 6-speed is the way to go. Actually, for "mild hills" (whatever that is), a 2-speed may be totally adequate if you have the corresponding fitness.