Legitimate reasons include illness or other compelling reasons for absence. The university decides on a case by case basis what is considered a valid excuse for absence. Reasons for absence apart from illness are only approved under special circumstances.
The absence must be documented with a doctor's confirmation or another type of document which must be dated the date of the examinationat the latest. In order for a student to demand a postponed examination, the documentation must be delivered to the university or posted at the latest seven days after the examination is arranged.
If you have two examinations scheduled for the same day, you can apply for a legitimate absence for one of them, provided that you have passed all the prerequisites if any.
If you have been granted a legitimate absence, this will not count as one out of three attempts. And you also have the possibility to sign up for the postponed examination if you if want to (please note that you need to sign up yourself in that case, it is not done automatically for you).