It's just personal preference, but I'm not a fan of Brompton's shifters. Although it's very convenient to shift, I feel that it's too flimsy, plasticky. When I hit some bumps, I sometimes hear it rattles. So I started researching and asking friends what replacements I can find.
3 speed shifter. I purchased this Sturmey Archer SL-S30 3 speed shifter for internal hubs. This version shown in the pic is a nov-design modified version of the SL-S30 with a small nut that holds the cable head. Sturdy and classy look is just nice.
One issue I have with this is the stiffness of the shifting. It is not as soft as the stock shifter but I'm getting used to it.
Installation is quite straight forward. Just remove the stock shifter and replace with this, insert the shifter cables, adjust, test and ride :)