Something Wiccan This Way Comes by Emma Harrison (2003, Paperback)

Sister Wiccas gather round.
By our power we are bound.
Hand in hand and heart to heart,
force all evil to depart!

Paige Matthews is new to the whole "Power of Three" thing, and looking to explore her powers. So when she reads of a Wiccan convention taking place just outside of Las Vegas, she thinks it's a great opportunity for her and for her sisters to learn more about their witchy ways. Though the girls are all over saving innocents, they aren't that heavy into Wicca practices, and Paige thinks the retreat could be enlightening.

Piper and Phoebe, however, aren't so sure. They're certain that while they spend their days battling honest-to-badness black magic, their so-called supernatural sisters will be nothing more than a bunch of blessed wanna-bes. Soon though, they hear of a rash of murders committed against practicing Wiccans, and making an appearance at the convention becomes top priority. But as they head for the desert campgrounds, are the Charmed Ones headed straight for disaster?