Triquetra

The Triquetra is an ancient Indo-European three-pointed symbol representing three-way unity. Sometimes it has a circle running through it and other times it does not. The circle in many belief systems is a symbol of eternity. For Pagans/Wiccans, it represents the Triple Goddess as the Maiden, Mother, and Crone.

The Triquetra is the thematic symbol of both Destined and Charmed. The Triquetra represents the Power of Three/Charmed Ones. It can also be found on items such as on cover of the Book of Shadows, the Spirit Board, Kit's collar, and on numerous other items

Destined and Charmed
The Triquetra can be found on the cover of the Halliwell Book of Shadows, on the Halliwell spirit board and on the collar of Kit, the original Charmed Ones' pet cat and familiar. Many covens and individual witches use the Triquetra as their symbol, though it is mainly used as the symbol of the original Charmed Ones' bond through the Power of Three and Warren Line of witches.

A Triquetra is sometimes used as a portal from one plane or timeline to another, such as in the Time Travel spell Chris (dark future) and Bianca (dark future) used to go both into the future and into the past. The original Charmed Ones used The Power of Three and Dominus Trinus to bridge the gap between the Astral and Earthly plane.

Trivia

 * The triquetra is also believed to represent the three planes of existence - Physical, Mental and Spiritual. Notice how, although the three points are clearly separate, they are interlocked together, symbolizing the connection between the three planes--and three sisters.
 * The triquetra can also represent the Past, Present, and Future. The tripartite division of the world into the realms of Land, Sea and Sky and the cycle of Life, Death, and Rebirth.
 * The first person shown in the series, Charmed, a witch named Serena Fredrick, who was murdered by the warlock, Jeremy, had the triquetra tattooed on her body. There were also two other victims who Inspector Andy Trudeau believed to be witches. Some fans speculate that these witches were somehow tied to and supposed to help, lead or guide the Halliwell sisters in their discovery of their powers.
 * Latin Tri- "three" and Quetrus- "cornered".
 * The triquetra might also be connected to the three Fates (Clotho, Lachesis, and Atropos).