Originally Posted by RegularGuy
You'll need to replace your bottom bracket, front dérailleur, rear dérailleur, crankset, and at least the front shifter. Might as well toss a new chain on and replace some cables while you're at it.
It won't be cheap.
I suggest you call your LBS or check out some of the online vendors to find prices.
OR an alternative, and much cheaper than going to a triple and darn good, is to replace the rear dérailleur and cassette for a Shimano XT MTB rear dérailleur and cassette. That set-up works better than good, costs less than 1/4 than replacing almost your entire drivetrain. The rear shifting is not quite as precise as with the Dura Ace set-up, but then nothing else is, but it works very well.