I'm assuming standard drop bars. What groupset are you using?
Do you have a "compact" crankset? 34x(something) 110bcd?
Not all of the current Shimano groupsets support a 3x option. Campagnolo generally does allow 3x for their shifters.
But, your change is likely going to take at least:
- New Crankset
- New Front Derailler (cyclocross, not MTB)
- New Left Shifter