The length of time it takes to complete your thesis will vary from student to student. This is because people work at different speeds, projects vary in how difficult they are, and some projects produce more problems than others. To finish in three years means that you will need to work very hard and that you will not have any major problems to overcome.
The viva examination usually takes between one and two hours. In some universities in addition to yourself and the two examiners there may be a senior member of academic staff present to act as chair of the examination. Your supervisor may also be present, although some universities do not allow this. However, even if there is a chair or your supervisor is present, they are not allowed to ask you questions or to take part in any of the discussion about the outcome.
