The three shells used in the toilets even confused poor ol' Sylvester, how were they used?
By 2032, toilet paper has fallen out of use; a set of three seashells is provided in every toilet stall. Though their method of use is left unexplained in the movie, Stallone later suggested a possible method in an interview as told to him by one of the film's writers involving extraction with two and scraping with one.
This is the page with the interview to Stallone where the question of the questions was originally answered. We all say thank you Louis Saucedo from Dallas, Texas for having asked it exactly that day.
Here is also a small screenshot for posterity.