Last summer i read these books and i can't remember the names can someone help me?

You are thinking of the Study trilogy by Maria V. Snyder: Poison Study, Magic Study, and Fire Study (which I have probably put in the wrong order). I just read two of the books in their spin off trilogy, known as the Gl trilogy, and I am VERY sure you're talking about the Study books. The main girl was named Yelena.