I read a book of this series a long time ago. It wasn't the first in the series. There were three main characters I think. I don't remember the plot or the motive but the characters were going somewhere to defeat someone and it involved going through a number of worlds. The book was about their journey to one such world. I'm not really sure but I think two were boys and the third was a girl. I think one boy's name started with 'F'.
I don't know the book's name but I remember that for some reason the title reminded me of Harry Potter (I don't know why.)
There were also trolls in the book.
The characters could use some sort of magic or atleast one could. I remember one of them ignited a magical fire (I think they were in cave at the time) and the fire burned much brighter than expected. The reason as one of the characters told was that they were getting nearer to something (their destination maybe?) and therefore the magic was getting stronger.