In regards to who serves as the cutoff for a ball hit into left field, with 1 exception that we’ll talk about in a second, it’s usually faster for the SS to be the cutoff. That’s because the SS will usually be going after many of the balls hit out into leftfield or leftcenterfield so it’s just easier for her to stay out there and act as the cutoff on all balls she couldn’t reach. (As opposed to her trying to get to a ball hit into LF or LFC and then racing back to cover 3rd and the thirdbaseman then going out for the cutoff) Now, the exception is the ball hit directly down the thirdbase line. This ball is easier for the 3rd baseman to go out on and act as the cutoff and the SS to cover 3rd – especially if it’s a ball hit in front of the leftfielder. I recommend that you use whatever coverage is easiest for your players to do. There is no ONE WAY to do this. There are “common ways” but that’s usually based on the typical player. If you have an exceptionally fast footed and smart 3rd baseman and your shortstop isn’t as quick, the use your 3rd baseman as the cutoff, otherwise, what I mentioned above should work for you.