The Power of Three

The Power of Three is a strong theme throughout the series run of Blessed and its mother series of Charmed; in Charmed the three Main Characters, Piper, Phoebe, and Paige; The Charmed Ones, are connected through the Power of Three. In Blessed, along with all of the Blessed Ones having a psychic connection with each other, each of the Charmed Families are stated as having "inherited" the Power of Three from their Mothers, though theirs are much less powerful.

As stated in Charmed, the Power of Three is easily one the strongest forms of Circle Magic in all of existence. There also exist more augmented forms of the Power of Three known, as the Powers of Six and Nine, and a sister power known as the Power of One and the Ultimate Power.

The Triquetra is the theme symbol of Charmed and Blessed, representing the three siblings (excluding Wyatt from the Halliwell Family) who make up each of their of their family's Power of Three's. This symbol is on the Book of Shadows.

While the more powerful spells written in the Book require either the Power of Three or some variant to truly work, it isn't always necessary for every member to cast the spell together; they just need to be in the same room/area/space at the same time. Each of the Halliwell sisters have shown different examples of this in the past in Charmed, and the Blessed Ones have also shown this to be true on multiple occasions.

Origins
Nobody knows just how exactly a Power of Three bond is formed, nor why it's usable only to the members of the Warren Line. But its key factors seem to be prophecy, trust, and identity, as well as lineage and relation. For example; when Tori Hartwell lost her memories, she temporarily lost the ability to cast any sort of Power of Three spell. It has also been implied that an oath or promise of some sort is also important, as the Blessed Ones all made a promise to always stay together through thick and thin. It may 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, 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!

Years later, Melinda Halliwell rewrote the spell to unlock the destinies of the Blessed Ones and give them the boost in power they needed to defeat the Shax Demon.