Thirty-five years ago it was fairly common for a woman to receive an enema as she went into labor--not a great experience. It is normal to pass some stool during the pushing phase of labor. It's important to avoid being constipated or having hard stool that can slow down pushing. My advice is to stay well-hydrated. In this hot weather 10 to 12 cups of water, juice, soup, jello or other liquid is important.
With your third baby the pushing phase may go quickly and it is possible that you won't pass any stool.
Wishing you a safe and happy birth experience with this baby!