One situation coaches see too often at the youth level is when a third baseman holds a runner at third base, similar to the way a first baseman would. That’s not the way you want to be positioned with a runner on third base. You need to focus on the batted ball and not worry about standing right next to the base. Be ready to field the ball and play good defense.
Coaches should encourage you to get in your normal position to be ready to make plays on a ground ball, pop up or a line drive. However, once the ball is clearly past the hitter, then come over to third base in case the catcher wants to make a throw. You also want to come back to the bag to make sure the runner is coming back to the bag and not running home on a throwback to the pitcher.
Defense is the priority here and it is important to make sure the runner is aware you are there, but the focus should be on the batter and being in good fielding position.