<P> 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 . </P> <P> 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 . </P> <P> 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 . </P> <P> 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 . </P>

When do the charmed ones get new powers