Alright, I'm posting this answer, but credit goes to Frock for finding it first.
This is the Tales of the Nine Charms trilogy by Erica Farber and John R. Sansevere, consisting of Circle Of Three (2000), Secret in the Stones (2001), and Islands of the Black Moon (2004).
Circle Of Three:
The first volume in a thrilling new fantasy trilogy in which the
characters must fight evil using only their wits . . . and nine
Walker, ever the class clown, reaches into a fountain and finds the
joke's on him when he wakes up in a distant world.
Niko wants to be a warrior, but his master insists he learn the ways
of a philosopher. What's the good of that?
Aurora can do simple gypsy magic tricks, but even she doesn't know the
true depths of her power.
Drawn together by forces they don't understand, to combat an evil they
can't imagine, their only weapons are nine simple charms.
Secret in the Stones
Zoe is living the life of a princess in a world she never knew
existed, far from her father and stepmother and her California school.
The ice-blue charm she found in a fountain back in her old
neighborhood must have something to do with her new life and her new
good luck. So she’s in no hurry to give up the charm, especially
because she thinks it might help her get home–when she’s ready to go.
But Zoe’s pretty blue trinket is one of the legendary Nine Charms and,
as its bearer, she is an unknowing participant in an ancient struggle.
If she ever hopes to see home again, Zoe must navigate a world of
magic that she barely understands. Her every decision will alter the
delicate balance between good and evil. And unfortunately, Zoe doesn’t
always choose to do what’s right if doing what’s wrong suits her mood.
Islands of the Black Moon
Now that you have reached the magical age of 11, it is time for your inheritance. . . .
So begins the note Lila da Gama receives from a distant aunt, and so
begins an adventure that will take Lila much farther than Tiger Lily,
her aunt’s crumbling estate. She’ll travel to another world, the
Islands of the Black Moon, the place where Lila suspects her father
disappeared six years earlier. There she must face terrifying
creatures, including a beautiful but evil sorceress, who are all after
one thing–Lila’s inheritance. Lila refuses to relinquish her precious
birthright, until the sorceress says the six words that Lila cannot
resist: I know where your father is.
With a little magic, a little science, and a lot of courage, as well
as help from an island boy, this spunky heroine sets off to change her
family’s fortune and bring her father home.