The Three-Faced Goddess is a supernatural being worshiped by the Ironteeth Witches. Could be a reference to the Greek goddess Hecate, also called the Three Faced Goddess. Hecate is the goddess of magic and the crossroads.
The Three-Faced Goddess represents the Mother, Maiden, and Crone. The Mother is represented by the Blackbeack Clan, whose High Witch is always a witch in her prime. The Maiden is represented by the Blueblood Clan, whose High Witch is always a youthful witch. The Crone is represented by the Yellow Legs clan, whose High Witch is always an Ancient.
The Three-Faced Goddess is commonly represented by a witch symbol. SPOILERS: This symbol is found in both the Lock, and the Eye of Elena.