what is the name of this book?

I cant remember the name of this book. It is about a girl who is adopted that learns that she has a twin sister named Gretchen. She finds out that she is a descendant of Medusa and that she has a third sister out there, making her a triplet. Those three are supposed to fight monsters and all that. I cant remember the names of the other two sisters or the author. I read this a while ago. The third book to the series was supposed to come out this year. so give me your best guess. Thanks