This is the triquetra sign, used by the makers of "Charmed" and has become a real logo of the show, though it isn't invented by them. It is a pretty old symbol that has different meanings in different religions and cultures . They took this because it was something different then the always token pentacle symbol. The triquetra is also on the cover from "the book of shadows" and on the bottom of the ouija board the Halliwell sisters have. The symbol is old, and can be retraced in the christian bible. Because of this, there excists a lot of Christians who are mad to the producer of Charmed for stealing their symbol, but the symbol excisted already 700 years BC so these Christians have no right to say anything about stealing. Towards this, you have the "Wiccans", real witches now these days, or the Pagans in the past, who used this sign earlier.
Triquetra stands for "try-KET-ra", latin for "three cornered".
In the Christian symbolisme, the triquetra stands for the holy trinity : The Father, The Son and The Holy Spirit. The unbroken circle stands for Eternity. The woven kind of the symbol applies to the indivisibility and equality of the Holy Trinity. It symbolises that the Holy Spirit co-excists of three excitations : honour, pride, and power, but is indivisible one God.
In the Wicca/Pagan/Godess symbolisme, the triquetra stands for the three-woven nature of the Godess : Virgin, Mother and Woman. It symbolises Life, Death, and the Incarnation of the three forces of Nature : Earth, Air, and Water.The inner circle stands for the feminin element and fertility.
Many cultures and religions do see the number "three" as holy and divine. The symbol of three grabbing circles, has also been found on 5000 year old indian religious statues. The triquetra symbol itself has been found on carved stones from the eigth century in Northern-Europe. The resemblance with the 'Valknut' is very big, the valknut was the sign for the Norwegian god Wodan and stood for the complexity of the human race and the infinity of live. The discussion who used this symbol first is quikly solved. The Paganism existed already 700 years BC, the believe and the symbols where already in use than. People founded different rune stones with this symbol in Sweden, Denmark and Norway.
But finally it is not that important to know the origine. The meaning behind this symbol is much more important. In every religion the sign stands for "Good" and this is where the "Charmed-Ones" believe in.
Only the good can save the world from the bad. And sure that this is necessary. But if the symbolisme behind Charmed gets through to the viewers is the question.