Also from KnowYourMeme:

On December 5th, Redditor Uncleluke2 created a post on /r/movies claiming he heard an explanation of the three seashells during a Q&A session with Demolition Man screenwriter Daniel Waters. "I won't tell you the actual secret, but I'll tell you where it came from. There's a scene where Stallone has to use a restroom. I'm trying to come up with futuristic things you'd find in there. I was having trouble, so I called my buddy, another screenwriter across town, asked him if he had any ideas. Ironically enough that guy was taking a dump when he answered the phone, looked around his bathroom and said 'I have a bag of seashells on my toilet as a decoration?' I said 'Ok, I'll make something out of that.'"