Its best not to book any holidays when uni starts, but in your case you didnt know.
Many people miss lectures for one reason or another. Sometimes a couple or practically all of them - but if your lecturer puts the notes on webct- that could cover you.
Also, are you missing 3 days of classes for each subject or 1 or 3 weeks? and are you going to be mssing more than 1 subject?
See lectures are not usually compolsury although in some attendance is recorded. However, if you are missing tutorials, these are compulsury- you can miss only two classes that are unexplained.
Also, depending on when in the semester you go because you may have pieces of assessment due, class presentations and mid-semester exams. But if its the first week or second week you should be fine.
Also, if you are not aware we do have a mid-semester break from the 25-29 April and the easter public holidays from 25-28 March.