Final answer:
Upon the command "Open Ranks," members of the third rank are required to step back, which maintains alignment and distances between the ranks in a military formation.
Step-by-step explanation:
When the command "Open Ranks" is given in a military drill instruction, members of various ranks are required to take specific actions to create space within the formation for inspection or to prepare for another movement. For members of the third rank, the appropriate action to take when this command is given is to step back. By doing so, they maintain alignment while adjusting the distance between ranks as instructed by their drill leader. This movement is critical to maintaining the order and discipline essential in military formations.