Avalon question

This is most likely an easy answered thing (I hope I am posting this in the right place.)
I recently finished the 13 books in the Avalon Web of magic and Quest for magic book series. I’ve now discovered the Warlock Diaries Manga. I am slightly confused though. So the Warlock Diaries has the three books and the Omnibus.

When I look online at the description of the Omnibus and the description of the first book of the Warlock Diaries they are the same description. So do I read the three books and the omnibus, or is the Omnibus all three books together? Or? I’m confused...