Lefty Slider

If you can master those three pitches... I mean so much so that if someone said you had to throw all 3 pitches to a certain location or your loved one would die (extreme I know, but still), and you can do it easily, then there is no reason to add another pitch.

The point I'm trying to make is, worry about those three pitches. A great fastball and a pretty good change up can get you pretty far, add a pretty good curve to that mix and you're even further.

Now you can say "Add a slider to the mix and I'll be even further yet!" Which is true, but it takes quite some time to be able to throw the pitch in any count with supreme confidence, so worry about those 3 first, and if.. some years down the road, when you've got them to go anywhere you want, then possibly think about adding another pitch.

But, it's your life, you can do whatever you choose to do

Sliders many times get developed because the pitcher didn't have a good feel for his CB and couldn't either get it to bite or throw it for a strike. If you are looking for something with a little movement experiment with your FB grips. 3 good pitches is plenty, many get by with just 2.