There's an AM session and a PM session (9am and 2pm i believe..).
Very rarely you may have two exams in the exact same slot, there are procedures for this. They basically lock you down so you can't ask anyone questions about it, alternatively, it can be moved (but you'll get a different exam to everyone else, for obvious reasons).
Since its rare to have two exams on the same day, let alone two exams in the same slot, to get three exams, you would need to hit *both* these unlucky coincidences. I seriously doubt it'll happen to you. I've never gotten a really horrible exam timetable myself. It can be distributed over 3 weeks so usually with 3-4 subjects, theres at least some room (but in this case, being unlucky is fairly normal).