I like your thinking but I imagine that the 3 Seashells are re-usable otherwise wiping your shit-stained ass would be costlier than dragging your butt across a deep-shag carpet. It is the future so there will be no waste whatsoever.
I think the seashells are magic. You put the first one upsidedown in the toilet bowl, shit on it, then put the second one on top of the first making a shit-clam-shell then piss on the third whilst rubbing it on top of the second.
This action makes all three shells shed their faecal matter into the ether and become sanitised for a further use, henceforth the user re-places them upon the shelf next to the toilet.
Job done
