You should list what you have on the bike now, but if it's a Shimano STI road drivetrain, you'll need...
Triple crankset
Bottom Bracket
(probably) Front Derailleur
(maybe, probably) Rear Derailleur
STI levers work with doubles or triples. If you have a short cage "double" road rear derailleur, it probably won't take up enough chain slack to work right with the smallest chainring of a triple. Even moreso if you plan to go to a wider-range cassette.
RichC
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Training: 2002 Fuji Roubaix Pro (105 triple)
Commuting/Daytripping: 2001 Airborne Carpe Diem (Ultegra/XTR, touring wheels)
Commuting/Touring: 2000 Novara Randonee (Sora/Tiagra/LX, fenders, lights)