Some Bridgestones and possibly Miyatas as well were drilled and tapped for a third chainring and their front derailers and shifters have enough travel built in to make it relatively simple to do as long as you can find a chainring with the right BCD mounting.

I'd have to check my NineTwelve when I get home to see if it is set up that way.
http://www.miyatacatalogs.com/2007/1...alog-1984.html