Power of Three (Charmed)

The Power of Three, on The WB series Charmed, refers to any bond formed by three magical beings good or evil. It is most commonly used to describe the bond between the Warren/Halliwell line, more specifically the Charmed Ones, which is said to be the most powerful magical force in history. The "Power of Three" originally included the Halliwell sisters Prue (Shannen Doherty), Piper (Holly Marie Combs) and Phoebe (Alyssa Milano). However, following Prue's death in the third season finale, their long-lost younger half sister Paige Matthews (Rose McGowan) takes her place within the "Power of Three" from season four onwards.

To my three beautiful girls. May this give you the light to find the shadows. The power of three will set you free.

It is not explicitly stated how a "Power of Three" bond is formed however. Melinda Warren – the first witch seen in the Warren/Halliwell line prophesied the birth of The Charmed Ones when she was burned at the stake, "With each generation, the Warren witches will grow stronger and stronger – until, at last, three sisters will arrive. Together, these three sisters will be the most powerful witches the world has ever known."

The "Power of Three" refers to the combined powers of the Halliwell sisters which is noticeably stronger than their individual powers. During the series' run, the sisters are able to protect themselves against powerful spells by destroying many demons and warlocks when using their combined powers. This can be seen in the first episode as the sisters battle a warlock and are able to protect themselves against fire and destroy him by chanting "the power of three will set us free". While some spells require the "Power of Three" to take enough effect, it isn't always necessary for all three sisters to chant a spell together, only as long as they are all together at the same time.

This power was also shown to connect the sisters on both emotional and magical levels. Certain spells cast on one member can affect the others. The most notable instance of this occurred in the season three episode "Bride and Gloom", when Prue (Shannen Doherty) is turned evil by a spell that eventually spreads to both Piper (Holly Marie Combs) and Phoebe (Alyssa Milano). Also, in the season three episode "Power Outage" a demon manipulates the sisters into fighting and using their powers against each other resulting in them losing all of their magical powers. It is only through mending their relationship that they are able to retrieve their magic. It is also revealed that the sisters' powers are greatest when they are in their Halliwell Manor home with the Book of Shadows. The "Power of Three" is represented by the ancient symbol called the triquetra. An embossed triquetra is placed on the cover of the Book of Shadows and splits whenever the "Power of Three" is split.

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Wikipedia