What I remember about the show is that some teenagers (3, I think) were some kind of monks that were able to transform through a device (I don't think that it was a power they were born with) into multiple monsters and could transfer monsters from each other.
I remember some good vs. evil stuff and that there were some tournaments between the group, I remember that one was in a desert for a powerful sceptre (?) and that the main antagonist had the monster that belonged to the main character's father before he died/disappeared and that the main character retrieved that monster during the tournament in the desert.
The cartoon was in English but, I watched the cartoon about 5 years ago but I think that it aired later in my country and I can't remember if it was airing for the first time or not.