Power of Three (Charmed)
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.
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 were able to protect themselves against powerful spells and destroy many demons when using their combined powers. This can be seen in the first episode as the sister's 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.
