Power of Three

The Power of Three is the generic term for any collective of powers that consists of three magical beings.(The Charmed Ones) are connected through The Power of Three. This type of magic is said to be the strongest that has ever, does and will exist. The Triquetra is The Charmed Ones symbol of their Power of Three.

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) who, when their circle was complete, would invoke The Power of Three.

Pros & Cons
The effect of a Power of Three bond being formed is that all the members' powers are pooled together into a Collective.

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. Matt and his sisters have been shown to have taken direct hits from all kinds of attacks such as fire and energy balls and live many times over whereas such attacks would vanquish any other magical creature instantly.

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 Mai's abilities were boosted during the song 'Play With Fire'. Matt's abilities spread to cover a distance up to Sydney, while Megan was capable of compelling others simply by talking.

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.

Another way, mentioned above, would be if one of the members died. Although this event is less likely the implications are more severe as there isn't really a way of coming back from that one (except under special circumstances, for example, Paige).

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 once mentioned that her and her sisters once took one to always remain friends.

It may also be that one needs the aid of a spell to create such a bond. Phoebe herself used 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!

In the case of the Heirs, their Power of Three Bond stems from the Charmed Prophecy, as well as similar experiences in life. The bond of The Heirs is primarily based on their mutual experience of having their powers blocked, and then restored, as well as their mutual sibling bond.

Due to their age, The Heirs have been shown to add members to their Collective temporarily, usually a friend or their Mother. It is theorised this is allowed through Matt's powerful gift of Empathy, allowing him to bring others to the same level as his siblings and keep them there for as long as he desires.