Short, Mid, Deep is one of several drill ideas contained in a recent TennisPro Magazine article by PTR Master of Tennis Oliver Stephens.
Players play singles and are challenged to hit 10 balls in a row short to short. Then 10 more mid court to mid court, and finally 10 more baseline to baseline. Ideally they will get to the point where they can use one ball and keep this pattern going for a long time. It is not spelled out in the article but I think Oliver intends that the players do not stop between transitioning from short to mid to deep and back again.