It is their symbol of friendship and their against ridding the world of WANNABEES.
(this also counts for vicky's Jayne's and Helen's in austraillia and france etc)
The Power of Three is symbolized by a "triquetra", a Celtic pattern that shows the center of three connected circles (somtimes bound together by a fourth circle in the center).
1. The Holy Trinity (Father, Son, Holy Spirit)
2. Body, Mind, Soul
3. Prue, Piper, Phoebe (original) or Piper, Phoebe, Paige (reconstitution)... and in the future, Wyatt, Chris, Melinda (The two sons of Piper Halliwell, and the daughter of Phoebe Halliwell).
Note: The future daughter of Phoebe Halliwell will be named Melinda, in honor of their ancestor Melinda Warren. Piper Halliwell was supposed to give birth to a female witch, who was to be named Melinda. But after her path had been changed, she ended up giving birth to two male witch/white-lighter hybrids, Wyatt and Chris. Therefore, Phoebe used the name Melinda for her own daughter.