The third baseman or pitcher, whoever does not field the ball should peel back and cover third but if they fall asleep... if your runner from first base would like to race my left fielder to third I am game if my LF loses I will go find another one. I ALWAYS tell my outfielders on every play if you are not moving somewhere you are doing it WRONG. If it is the most routine of nothing plays you should still be running toward the infield just in case.