Hmmm, it may be possible that one of your fish decided to do this, or that something happened with your water parameters that caused the snails to detach from their shells, which were then shredded once they were no longer protected by a shell... but I'm guessing you don't have a complete list of the animals in your tank. I suppose you'd know it if you had a hitchhiking octopus in your tank, but they tend to do exactly the things you described: they will pull the snail from the shell, "shred" it and partially eat it, and leave pieces of the dead animal scattered in your tank. There are several other predators that may extract the snails, but few that would extract three in one day. If you discover what's killing the snails, please let us know. (Check at night, with a flashlight, and see if you can't catch the snail killer out in the open.)
Cheers,
Don