The information in this article is from the fan game King's Quest II: Romancing the Stones and is therefore not canon.
The three stones of Growth (Air), Birth (Water) and Death (Earth) that would open the Door of Destiny in Kolyma. It was the only way that Graham would travel to the Tower Realm.
In the official KQ2 game, the stones are Magical Keys used in the Magical Doorway.