So you are going to wear some kind of pants that you can’t take off easily for 3 days. Maybe some kind of medieval armor, or a fursuit.
Gatorade has tons of calories because of the sugar, and it will leave nothing to poop out. It will give you the critical electrolytes (plants crave them) that you’re missing from food. Take a multivitamin too.
But it doesn’t have enough calories, and you need stuff to go in your stomach so you don’t suffer being empty on your temporary Gatorade diet. eat white bread. Your body metabolizes almost all of it, so there’s almost nothing to poop out.
You will need to poop out all your poop before doing this, so good hydration and veggies beforehand will clean you out. No need for laxatives, just switch to your new and stupid diet 36 hours before you need to stop pooping.
Good luck on your medieval battle reënactment or orgy.