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, does and will exist.

The Triquetra is the theme symbol of Charmed, representing specifically the Charmed Ones themselves and their Power of Three, 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, only 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; Piper has summoned the Power of Three to save Paige from an invading spirit; 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 one must take a blood oath to always remain together or something of that nature as Phoebe mentioned that she and her sisters once took one to always remain friends. (The Witch is Back)

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!

Triangle of Power


Evil beings have also been seen to exploit the Power of Three. For example, the Rowe Coven were a trio of two warlocks and a half warlock (just like the reconstituted Charmed Ones).

Pros & Cons


The effect of a Power of Three bond being formed is that all the members' powers are pooled together into a Collective. One of the benefits of this bond is High Resistance.

Of course, the downsides of this are that the amount of power one gains from this is relative to the level of the other beings involved, with the above example assuming that the beings involved are (at least more or less) the same strength.

Another downside of this is that if one member dies it weakens the Collective, making the other members easier to kill off with the death of each member. However, on the flip side, more powerful (and hence higher level) beings are harder to kill/affect with ones powers than lower level ones i.e. Piper and the other various Charmed Ones have been shown to have taken direct hits from all kinds of attacks such as Fireballs and Energy Balls and live many times over whereas such attacks would vanquish any other magical creature instantly. The Triad are also highly resilient to Piper's combustion power whereas other lower level demons usually disintegrate within seconds. Therefore this particular con is easily balanced out.

Then there is the fact that this collective interconnects all of the members to each other in various metaphysical ways. Certain spells cast on one member may affect the others through this interconnection such as when Prue was magically turned evil, causing Piper and Phoebe to turn evil also.

A Power of Three bond can be broken in several ways. If the members use their powers on each other in a negative manner or atmosphere then the bond will be broken. Fortunately, this may be reformed by all of the members reconnecting with each other emotionally or something to that effect. For example, when the Charmed Ones used their powers on each other during an argument it shattered their Power of Three. However, later on they all got over the argument and showed their solidarity as sisters (which is where their Power of Three comes from) and hence reformed it. (Power Outage).

Another way, mentioned above, would be if one of the members died. Although this event is less likely the implications are more severe as the collective would be weakened beyond repair (excluding the unlikely addition of another member, for example, Paige Matthews).

The Power of One
The first time that the term "Power of One" was mentioned was on the episode The Wendigo. Also the Power of Three was once referred to as the "Power of One" because all three sisters work as one.

The Power of Four
This is a real Charmed-world term and was used by the Phoebe of an alternate Universe where the balance of power was shifted towards evil. The term described the collective powers of Paige, Phoebe and their evil counterparts from the alternate universe, and is shown to be weaker than the Power of Three, not being able to finish Gideon off.

Many Charmed fans also theorized that had Prue, Piper, Phoebe, and Paige ever all been alive and met and connected with each other, that it would've led to a Power of Four bond between all of them.

However, Prue reveals in the Season 9 issue The Heavens Can Wait that if she and her three sisters were to unite, they would be weakened drastically and it could possibly cause cataclysmic effects. This is because Melinda Warren never prophesied a Power of Four and therefore it should never have been able to happen. Prue later concludes that the only way for the Charmed Ones to exist is through The Power of Three.