It is highly recommended to talk about three books, but make sure that you are able to equally talk about all three. If you are going to focus on all three, then you must be comfortable with all three works. This is essential to a good paper as examiners will be able to identify students who clearly have more knowledge about two out of three or even one out of two works. If you don't feel like you can write about all three, then it is highly recommended that you solely focus on the two you feel the strongest about.However, do not go into the exam room having chosen two of the works, as there may be no questions that fit that combination.