3rd pitch/pace on how hard I could throw

Question
Hello I'am 13.5 yrs old 5.1 88 lbs I throw a 68-73 mph fastball and a 60-65 slurve. I have trouble finding a 3rd pitch i experimented the change up/ cutter did not work.
I also have not hit a big growth spurt yet and I have the velocity of a 5-8 good velocity pitcher. I wonder how much harder I could throw and what is my full potential in mph when i'm 17. please help me with a 3rd pitch. I have a slurve that looks like it could fool hitters when they start to get aggresive

Answer
I can't say or I don't think anybody can tell you how hard you maybe throwing at age 17.I think you need to develop the slurve to a curve ball. I think with the change up you can try different grips. Look up the different grips on the computer and try them. You may want to try to throw a splitter as an off speed pitch.