If you want three pitches you may want to consider four-seam fastball, two-seam fastball and change-up. If you throw the two-seam with back spin you will get movement and a decrase is speed from your four-seam. Once you perfect it, you can get it to run in on a right handed hitter if you are a righty. The mechanics of the four seam, two seam and change up are the same. Honestly, you should do fine with a good fastball and change-up. I find that many kids your age who say the have a pitch can rarely throw it effectively. We faced a player from an opposing team last year who always bragged about his "killer" curve ball. Problem was he usually ended up beaning the batter (my son was a victim !) when he tried to throw it.
A cutter can be an extremely effective pitch but you don't need at your age to be able to pitch well. Focus on being able to master the location on your four-seam. This will inevitably lead to you being ahead in the count early. That is when pitching is fun. The pressure is on the batter and not you !