Join Date: Feb 2005
Bikes: Trek 5500, Colnago C-50
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You'll need a triple crankset with chainrings, new bottom bracket, front and rear derailleurs.
There is no way to just attach a third chainring.
There have been many forum postings from people trying to convert from a triple to a double. Maybe you could make a trade. Remember that the shifters need to match the cassette and chain, 9-speed to 9-speed, 10-speed to 10-speed, etc.
105 shifters are double/triple compatible, but the speeds must match the cassette and chain.