Struggling a little with the Nikon D3 custom autofocus settings for baseball. I am comfortable with the single servo (AF-S) but not so with the continuous servo (AF-C).
I would recommend using AF-C with the focus priority taken OFF of the shutter button. Either use the AF-ON or the other back button.. (Canon shooter here.. sorry if I don't know which button is which) for focusing.
This way, you get the best of both worlds.. you can single tap the AF-ON button to quickly focus on a static subject, or hold the button down to continuously focus as a runner is coming down the line.
This also helps incredibly for composing subjects off center without having to change your focus point.
Sorry if this is a little vague.. it's kind of hard to explain here.