You need a triple chainring crankset, compatible BB for the triple crankset, a left-side STI shifter for Triple (or a down-tube or bar-end friction front shifter in which case you would need a separate brake lever or could use your STI with no shifter cable--which would look kind of silly), long cage rear derailleur, triple compatible front derailleur, and probably a longer chain.
You could use a "triple-izer" front chainring set-up with the old crankarms but you still need everything else I suggested.
It would be much cheaper to get a wider range cassette or consider a compact crankset double set-up.