Think of University study not in years but completing a certain number of subjects (credit points) and making sure your subject selection meets the criteria for graduation (completion of core units).
So, for a typical Science single degree student, you'd complete 4 units x 2 semesters x 3 years = 24 subjects.
You can do up to two summer units (some faculties charge you CSP if you're a CSP student, others charge full fee) at an absolute maximum. Most do one. You can overload an unit per semester, with faculty approval. This generally does not happen until second year and also if you've maintained a certain (generally Distinction / Credit) average.
This will mean that you can complete your course about one semester quicker (1 unit x 2 summer semester (end of 1st and end of 2nd) and two overloaded semesters = 4 units). That being said, you're gonna be a zombie afterwards.
You can also take a reduced load and extend your degree. If you do 0.75 EFTSL per year (3 units a semester as Fyrefly is doing), your 3 year degree will take 4.
Like I say, the easiest way to look at course lengths is how many units/credit points you need to have. Then add on the fact that you may be able to overload a unit per semester after first year and take one or two subjects in the summer.