The Power of Three

The Power of Three is has been a strong theme throughout the series run of Charmed, as the three main characters, the Charmed Ones, were connected through the Power of Three. It has also been a strong theme in Destined, as three of the main characters became the new Charmed Ones. 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 and Destined, 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 spell 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. Prue Halliwell said this spell, with Melinda and Tamora beside her; allowing them to become the new Charmed Ones.

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 that is titled "Dominus Trinus". Prue Halliwell would later have to cast this spell to bestow the power onto the next generation.


 * 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, in Charmed, the Rowe Coven were a trio of two warlocks and one half-Warlock (just as the reconstituted Charmed Ones were made up of the witches Piper and Phoebe and the Whitelighter-Witch Paige), making them the evil equivalent to the 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, the ability to withstand levels of magical force that would kill an ordinary person.

Of course, the downsides are that the amount of power one gains from this bond is relative to the level of the other beings involved, though, in the case of Prudence, Piper, and Phoebe, they were all more-or-less equal in strength when they first received the Power of Three. This goes for Prue, Melinda, and Tamora when they first received the Power of Three.

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/effect with on'es powers than lower level ones; for example, 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 have survived them many times over, whereas such attacks would instantly vanquish any other magical creature. 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 by the strength gained by their collective. Prue, Melinda and Tamora have also easily survived direct hits from all kinds of attacks.

There is the fact that this collective magically links all of the members to each other in various metaphysical ways. Certain spells cast on one member may affect the others through this connection, such as when Prudence was magically turned into an evil warlock through a Dark Wedding ceremony, which subsequently caused Piper and Phoebe to turn into Warlocks as well.

Additionally, the 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 bond can be reforged by all three of the members reconnecting with each other on an emotional level, or something to that effect. For example, when the Charmed Ones intentionally used their powers on each other during an argument, it shattered their Power of Three. However, later on, they forgave each other for their actions, showing an understanding of the others' point of view and demonstrating their solidarity as sisters (which is where their Power of Three comes from) in a battle against a demon; as a result, their bond was restored. For the newest Charmed Ones, the Power of Three comes from their bond and solidarity as cousins, friends, and technically their own sisterhood.

The bond can also be broken in a more permanent way if one of the members dies. Although the strength of their power increases their chances of not being killed in battle, it is still possible, and such a death could have severe implications to the bond, since the collective would be damaged beyond repair. In the case of the Charmed Ones, this was miraculously solved by the revelation that they had a half-sister, Paige Matthews, whose lineage as a Halliwell witch (as well as Whitelighter heritage) allowed the Charmed Ones' Power of Three to be reconstituted. However, this event is incredibly unlikely under normal circumstances.

The Power of One
The first time that the term "Power of One" was mentioned was in the Charmed episode The Wendigo. The Power of Three was also once referred to as the "Power of One" to describe how all three Halliwell sisters work as one.

The Power of Four
This is a real Charmed term that was used by the Phoebe from 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 when they're not able to finish Gideon off by themselves. However, this may be at least partially due to the fact that there were only two Paige's and two Phoebes and no Pipers to finish off the trios.

Many Charmed fans also theorized that had Prudence, Piper, Phoebe, and Paige that if they had all known each other while Prue was still alive and had connected with each other, it would have led to a Power of Four and made them stronger than ever.