“Solution - Old man with good teeth and shaky hands”

Your odds are 1 out of 2. As the game is not magical, it is impossible for the stone to really "jump." As two empty shells will always be turned upside down, it is those two that will be empty. So you then have two shells to choose from. If you watch the old man's hands, you will be able to see if he moves the pebble to a different shell.