Charmed
The story begins when Phoebe finds the Book of Shadows after seeing the word "ATTIC" spelled out on the spirit board. She reads out an incantation which gives them their powers. They remained a threesome until their sister, Prudence (Prue), dies fighting the demon Shax. Shortly after Prue dies, Piper casts a To Find A Lost Soul spell and finds that they have a younger half-sister, Paige Matthews. The triad or Power of Three is once again complete. This continues until the seventh season. After vanquishing the demon Zankou and an astral projection form of themselves everyone believes that they are dead. They give up magic to have normal lives. But the magic is still alive. They are sought out to help a young new witch named Billie who is a college student wanting to fight demons. Paige is first drawn to her because she is Billie's Whitelighter.