The simple answer:
1) Loft: the angle of the clubface. Higher numbered irons have more loft than lower numbered irons. More loft means the ball flies higher and less far. Clubs like 8 and 9 iron are made for accuracy, not distance. An average golfer probably hits a 9 iron 130 yards, as opposed http://www.pga.com/home to hitting a 3 iron 180-190 yards.
2) Shaft length. A 3 iron is longer than a 9 iron, so the club head is moving faster at impact. More search engines clubhead speed = more distance.
So in your question, a 2 iron has slightly less loft and more shaft length than a 3 iron, meaning you will probable hit the 2 iron 10 yards further than your 3 iron.