The Power of Three, on the WB

The WB Television Network

The WB Television Network is a former television network in the United States that was launched on January 11, 1995 as a joint venture between Warner Bros. and Tribune Broadcasting. On January 24, 2006, CBS Corporation and Warner Bros...

 series Charmed

Charmed

Charmed is an American television series that originally aired from October 7, 1998, until May 21, 2006, on the now defunct The WB Television Network. The series was created in 1998 by writer Constance M...

, referred to any bond formed by three magical beings good or evil. It was most commonly used to describe the bond between the Warren/Halliwell line, more specifically the Charmed Ones, which was said to be the most powerful magical force in history.
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."

This also seems to continue throughout the family as each of the sister's has three children, and each of Piper's children will have three children.

Connection

The Power of Three refers to the combined powers of the Halliwell sister's which is noticably stronger than their individual powers. During the series' run the sisters were able to protect themselves against powerful spells and destroy many demons when using their combined powers. This can be seen in the first episode as the sister's battle a warlock and are able to protect themselves against fire and destroy him by chanting "the power of three will set you 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 when Prue is turned evil by a spell that eventually spreads to both Piper and Phoebe. Also, in the episode "Power Outage" a demon manipulates the sisters into fighting and 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 their powers are greatest when in their home Halliwell Manor with the Book of Shadows

Book of Shadows

A Book of Shadows is a book containing religious texts and instructions for magical rituals found within the Neopagan religion of Wicca. Originating within the Gardnerian tradition of the Craft, the first Book of Shadows was created by the pioneering Wiccan Gerald Gardner sometime in the late 1940s...

.The Power of Three, the ancient magic and bond that is imbued with the sisters, is represented by the ancient symbol called The Triquetra. An embossed triquetra was placed on the front of the Book and would split whenever the Power of Three was split.

First Generation

After three centuries, the prophecy was fulfilled by the birth of Prue, Piper, and Phoebe Halliwell. The sister's use their powers to protect innocent people and fight various warlocks and demons, often invoking their joint powers to defeat their enemies. Eventually, the sisters not only developed strength, but also willpower and an unusually strong bond with one another, enabling them to stand united despite their very notable differences. Their magic is rooted in their bond as sister's and as a family. This is shown through flashbacks in the episode 'Prewitched' as their grandmother struggles with whether or not to release their powers because the sister's do not get along.

Each sister's own powers grow as the series goes on to the point where they develop new ones; Prue develops astral projection, Piper gains molecular combustion whilst Phoebe is able to levitate. As these powers appear their joint powers as the power of three also appears to strengthen.

Second Generation

With Prue's death in 2001, the Power of Three was shattered. However, hope for its reconstitution was brought by the arrival of Prue, Piper and Phoebe's long-lost half-sister, Paige Matthews. Paige was the result of a forbidden, clandestine love between Patty Halliwell and Sam Wilder, a Whitelighter guardian angel. The Source; ruler of the underworld, planned to lure Paige to evil but needed to do so before she united with her sisters in their home and the Power of Three was reconstituted.

Once the three sister's are together again their magic and the power of three is alive again, and so they resume their fight against evil. Eventually Piper and Phoebe ask Paige to move into the manor. Although, she is at first hesitant, she agrees when she realises they safest and strongest when they are together.

The Book is also tied to their emotions which can both strengthen as well as weaken the Book. When the demon Zankou attacks the sister's he is able to steal the book and it is only once they sister's chant the Power of Three spell that they are able to regain it.

Other Variations

Th Power of Three has also been seen in other forms throughout the season. For example, in 'That 70s Episode' Patty; the girl's mother casts a spell with young Prue and Piper as she is pregnant with Phoebe at the time. Also, Penny (their grandmother), Patty, and Piper used the Power of Three to banish the Hollow in the final season.

Any three people possessing each of the Charmed One's powers through their blood or a 'To Call a Witch's Powers' spell can form their own Power of Three, provided they can stop the original users from channeling it in the first place. The Stillman Sisters exploited this for their own ends when impersonating The Charmed Ones.

The Triad was also a major Evil Power of Three.