The Power of Three

The Power of Three has been a strong theme throughout the series run of Charmed, as the three main characters, the Charmed Ones, are connected through the Power of Three. This is said to be the strongest kind of magic that has ever and will ever exist in this world.

The Triquetra is the thematic symbol of Charmed, which specifically represents the Charmed Ones themselves, along with their Power of Three, and which is marked on their Book of Shadows.

While most of the spells written in the book require the Power of Three to take enough effect, it isn't always necessary for all three sisters to chant a spell together as long as they are all together at the same time. For example, Phoebe cast the Dominus Trinus to awaken their powers as the Charmed Ones by herself and was merely in the vicinity of her other two sisters; Piper summoned the Power of Three to save Paige from an invading spirit; and Paige summoned it to vanquish a Slime Demon.

Origins
It is not explicitly stated how a Power of Three bond is formed, but prophecy, lineage, identity, and mutual trust seem to be key factors. It could be possible that the three must take a blood oath as a promise to always remain together, or something of that nature, as Phoebe mentioned that she and her sisters once took one to always remain friends prior to becoming witches.

It may also be that one needs the aid of a spell to create such a bond. Phoebe read a spell to bestow the power of the Charmed Ones upon the sisters that is titled "Dominus Trinus":


 * Hear now the words of the witches,
 * The secrets we hid in the night,
 * The oldest of Gods are invoked here,
 * The great work of Magic is sought
 * In this night and in this hour,
 * We call upon the Ancient Power
 * Bring your powers to we sisters three!
 * We want the power!
 * Give us the power!

Dominus Trinus
Dominus Trinus is a powerful spell that grants the Power of Three their respective powers. Phoebe Halliwell casted in 1998, giving she and her sisters their powers back.

Notes and Trivia

 * ny three people possessing each of the Charmed One's powers through their blood, or a "To Call a Witch's Powers" spell, for example, can form their own Power of Three, provided they can stop the original users from channeling it in the first place.