I had a 3x7 on my Moulton APB - absolutely loved it. Admittedly mine had matching brake lever / shifters for both the 7-speed derailleur and the 3-speed hub, so was very natural to use. It was great to be able to shift down at the lights easily, and much less hassle to adjust than an indexed 3-speed front derailleur. No worries about crossing chain from largest front to largest rear & vice-versa either.
Personally I'd go for an Alfine 11-speed given the choice (and the money) though, and get rid of derailleurs altogether.