An ‘open book’ exam means you are allowed to take books and other material into the exam with you. Any material found within the public domain can be taken in, which includes: books, presentation slides and course handouts. Any personal annotations in your materials must be kept to an absolute minimum.
A three hour exam sounds like a long time, but the amount of information you need to provide in your answers means you will need to be efficient in finding any information and writing your answers. Colour coded tabs or some other system of organisation will make it much easier to find information during the stress of an exam.
Remember, having a sufficient amount of information to reference during an exam will prove to be useful, even a life saver, but taking too much information in could be more of a hindrance.
It is crucial to practice with past papers to get the hang on the types of questions, the level of information required and the time it takes to complete each question. The exams at the University of Westminster had a choice of 8 questions, of which you would answer 5. Each exam had one question which was mandatory to answer. The compulsory question for the Law exam is typically about procurement routes and advising the client, and the compulsory question for the Practice Management exam is usually about the appointment and risks.
It is worth remembering that the multiple choice answers are worth the same amount of marks but are much faster to answer than the other questions.