QUOTE(desastar @ Dec 12 2015, 06:38 AM)
I have considered modding but it actually cost too much. SRAM dual drive hub will be at least US250-300 plus shipping, plus shifters.
If you have a contact to get cheap SRAM DD hub, please let me know as I'm interested to get one for my single speed bike.
Thanks!

When SRAM started, they don't have an internal gear division. They then took over SACHS. Those days, only Sturmey Archers and Sachs are the main players. Sturmey Archers is around even when I was riding a Raleigh Chopper 1977 (also 3 speed) and they lasted till today. After SRAM took over Sachs, the price skyrocketed because of their branding and not necessary because of their expertise. So, why limit yourself to a SRAM ?
Sturmey Archers actually have 5 and 8 speed (if I can recall). The 5 speed shares the same interior measurements with the 3 speed. That means if you can buy the 5 speed innards ( probably from UK ), you can fit it into a 3 speed Brompton hub. SO, that converts the 6 speed into 10 speed. Nobody here does that here. I have actually bought an old SA hubs and dis-assembled it and rebuilt it and it is actually not that hard. Alas, Brompton owners don't trust non-Brompton owners to do their precious bikes so I never service for anyone before.
I would rather go for the Shimano Alfine which comes in 8 speed (very cheap nowadays) but the 11 speed would cover more ratios. A cheaper option will be the Shimano Nexus which is 8 speed. Best option of all, I would say IMO

is to hunt for a used Rohloff in eBay.

14 speed which covers almost the entire 3 X 9 speed of the conventional FD and RD gearing systems. Also Rohloff are maintenance free!

Cheers!