YA/ MG Fantasy Novel Series where 3 children with different missions end up together to fight a cruel regime in a fantasy kingdom. One of the MC gets transported from modern world to the fantasy kingdom through a magical stone

[Lareb]

I read only one book from what was apparently a series in my school library way back in 2006-2009. The book may have been published before 2008 or even the 90s, The main characters in the book were 3 children, 1 girl and 2 boys. The girl and boy belonged to the fantasy world but the other boy finds a stone/gem on his school trip to a museum/zoo and ends up travelling from the modern world to the fantasy land. The story is told from the 3 different POV of the characters. The boy that travels from our world ends up in a military barrack type situation in an isolated desert type area, where he's made to undergo extensive training. The other boy was a scribe/page sent on a mission while the girl was either a mage in training or had magical abilities.
I don't exactly remember the picture on the book cover, but the cover page had a border made of drawings of different colored stones/ gems and the series name also had the word stone.
Please help me find this book/series if possible! . Any kind of suggestion or help will be much appreciated.

[Ayshe]

It seems to be solved on scifi.stackexchange.com: Circle of Three?

[Kate]

Lareb ~~
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