but how difficult is this to do and which triple would you guys recommend?

I have a 2007 Specialized Tarmac, and I love the bike, but age and bad knees have suggested that I add one more gear for my uphill runs. I don't want to buy another bike especially since I also have a Rivendell for most of my commuter rides, so I'd like the Tarmac to be a bit more forgiving on these Georgia hills when I trade off and use it as my commuter at times.