Forming a sound argument
Essay Question: With reference to three different factors, discuss possible reasons for the global success of Harry Potter books.
The thesis statement does not need to argue whether the books are a global success or not - that information was determined by book sales and is assumed in the question.
The thesis statement simply needs to identify the three different factors that you choose as the basis for discussion.
For example:
The global success of 'Harry Potter' books is due to the special book-launch, the film of the book, and widespread marketing of Harry Potter merchandise.