so your front derailleur is made to accommodate up to 3 gears but that doesn't mean it wont do 2. you will have to have the low limit screw adjusted, and your front derailleur shifter wont work properly. For everything to be kosh you will need a new front derailleur (specifically made for 2 chainrings), a new crankset with only 2 chainrings (your old one probably has the bottom 2 chainrings attached with the same bolts, if you remove one chainring then the bolt wont fit properly, a new 2x left shifter to replace the 3x shifter.