I was listening to the “How Did This Get Made” episode of this movie.  They mention that someone, somehow connected to this prop or idea or whatever, tried to give an explanation.  The explanation was dumber than I could have ever imagined in my many years as a child watching this movie and wondering this exact thing.  Dude said you somehow use 2 of the shells to spread your cheeks and the third to scoop/ clean.  Such disappointment.