I did "The Blood of Others" by Simone de Beauvoir and "Iron in the Soul" by Jean Paul Sartre and my topic was something along the lines of "the importance of setting and time in place to evoke...." I would most definitely do three different authors unless it was three very different books with a central theme such as Toni Morrison's writing, or Margaret Atwood. Also, it doesn't necessarily have to be three books. I did two, as did a couple of other people in my class, because firstly for me, it would have been ridiculous to do either of my books in 1,500 words alone - it was bad enough in 2,250 words. Secondly, a few of my classmates found that it was hard relating books to their topic and easier cutting out the one that didn't fit so well and concentrating on the two alone. The SQA's report on Advanced English said it wasn't necessary for candidates to do three anyway.
Advanced Higher English is an incredible amount of work, but for me doing the poetry of Plath, creative writing and the dissertation was one of the most stimulating and demanding courses I've ever done in my life - it changed me as a person, the way I think and work - and I'm very pleased I stuck with it.
And yes, I agree with Sophetta - if you want a life, and by that, I don't just mean going down the pub - I mean being able to read outside the syllabus, spend time preparing for university - do NOT do four Advanced Highers.