The Power of Three, on The WB series Charmed, refers to any bond formed by three magical beings good or evil. It is most commonly used to describe the bond between the Warren/Halliwell line, more specifically the Charmed Ones, which is said to be the most powerful magical force in history. The "Power of Three" originally included the Halliwell sisters Prue (Shannen Doherty), Piper (Holly Marie Combs) and Phoebe (Alyssa Milano). However, following Prue's death in the third season finale, their long-lost younger half sister Paige Matthews (Rose McGowan) takes her place within the "Power of Three" from season four onwards.
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To my three beautiful girls. May this give you the light to find the shadows. The power of three will set you free.
It is not explicitly stated how a "Power of Three" bond is formed however. Melinda Warren – the first witch seen in the Warren/Halliwell line prophesied the birth of The Charmed Ones when she was burned at the stake, "With each generation, the Warren witches will grow stronger and stronger – until, at last, three sisters will arrive. Together, these three sisters will be the most powerful witches the world has ever known."
Connection
The "Power of Three" refers to the combined powers of the Halliwell sisters which is noticeably stronger than their individual powers. During the series' run, the sisters are able to protect themselves against powerful spells by destroying many demons and warlocks when using their combined powers. This can be seen in the first episode as the sisters battle a warlock and are able to protect themselves against fire and destroy him by chanting "the power of three will set us free". While some spells 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.
This power was also shown to connect the sisters on both emotional and magical levels. Certain spells cast on one member can affect the others. The most notable instance of this occurred in the season three episode "Bride and Gloom", when Prue (Shannen Doherty) is turned evil by a spell that eventually spreads to both Piper (Holly Marie Combs) and Phoebe (Alyssa Milano). Also, in the season three episode "Power Outage" a demon manipulates the sisters into fighting and using their powers against each other resulting in them losing all of their magical powers. It is only through mending their relationship that they are able to retrieve their magic. It is also revealed that the sisters' powers are greatest when they are in their Halliwell Manor home with the Book of Shadows. The "Power of Three" is represented by the ancient symbol called the triquetra. An embossed triquetra is placed on the cover of the Book of Shadows and splits whenever the "Power of Three" is split.
First generation
After three centuries, the prophecy was fulfilled by the birth of Prue, Piper, and Phoebe Halliwell. The sisters use their powers to protect innocent lives and fight various warlocks and demons, often invoking their joint powers to defeat their enemies. Eventually, the sisters not only developed strength, but also willpower and an unusually strong bond with one another, enabling them to stand united despite their very notable differences. Their magic is rooted in their bond as sisters and as a family. This is shown through flashbacks in the season three episode "Prewitched" as their grandmother Penny (Jennifer Rhodes) struggles with whether or not to release their powers because the sisters do not get along.
Each sister's own powers grow as the series progresses to the point where they develop new ones; Prue develops astral projection, Piper gains molecular combustion, whilst Phoebe is able to levitate. As these powers appear, their joint power as the "Power of Three" also appears to strengthen.
Second generation
Following Prue's death in the season three finale, the "Power of Three" was shattered. However, hope for its reconstitution was brought by the arrival of Prue's, Piper's, and Phoebe's long-lost younger half-sister, Paige Matthews (Rose McGowan) at the beginning of season four. Paige was the result of a forbidden, clandestine love between the Halliwell sisters' mother Patty (Finola Hughes) and her whitelighter (guardian angel) Sam Wilder (Scott Jaeck). The "Power of Three" was then able to reform itself with Paige taking Prue's place.
Other variations
The "Power of Three" has also been seen in other forms throughout the series. For example, in the season one episode "That 70s Episode", Patty casts a spell with young Prue and Piper as she is pregnant with Phoebe at the time. In another occurrence, Penny, Patty, and Piper use the "Power of Three" to banish the Hollow in the final episode "Forever Charmed".
Any three people possessing each of the Charmed Ones' powers through their blood or casting the "To Call a Witch's Powers" spell can form their own "Power of Three", provided they can stop the original users from channeling it in the first place. The Stillman Sisters exploited this for their own ends when impersonating The Charmed Ones in the season six episode "The Power of Three Blondes".