What you can do is accept your first offer and then apply again in the later rounds. This might cost extra however.
It doesn't cost extra! Which is good, because it already costs too much in the first place.
This is the way it works (I believe): "University A" has 100 spots in Course B. 500 people apply for course B and only 100 get accepted. The 100th person to get accepted's ATAR becomes the new cut off.
Lets say though, that 10 people out of those 100 also applied through VTAC or something and got into a course they would prefer, so they turn down their spots. Lets say another 5 defer.
So now there are 15 spots left in Course B. In the late round there might be 300 applicants but they might not all be the original ones who all had ATARs lower than the cutoff. There may be new applicants with higher ATARs or just the ones with lower ATARs. It is my impression then that the 15 most eligible get accepted into Course B. So the admission rank for the course could go up or down.
All the university wants to do is fill all its spots with the best students possible.
If universities gave out offers to everyones top three, it is possible that they could be 3x over enrolled - or they would have to give their offers to less people which could result in them being very underenrolled. Also, giving multiple offers out would effectively raise the entrance rank, as the spots would be filled by those with higher ATARs.
Look guys, university isn't the be all and end all. If you don't like your course just consider deferment, or transfer. Its not like this one offer round is going to determine your future forever. Don't let it bring you down. See what you get and then make the best possible decisions.