3 3-hour exams in a row with no time in between. What can I do?
Whether this is a standard practice or not highly depends on your university. However, the attribution of these exam period is an automated process and some situations like yours are bound to happen. 

If I were you, I would consult the specific rules of your University. I know at that my previous Canadian University in Montreal (École Polytechnique), there was a rule that limited the amount of exam hours you could have in a 24 hours period, ( I believe it was 6 maximum per day). If such a thing occurred, you could ask for the exam to be moved as this was judged to be unfair.

You can also consult with your student committee (of your department or your faculty) and your department since you say that you are following a standard path. In that case, there will be many students in the same situation and they should prove to be understanding. The student committee can usually present a uniform front and defend the entire class as a whole concerning this issue. There is absolutely nothing impolite about this if you proceed politely and you act as soon as the semester begins. The sooner you handle this issue, the easier it will be to solve.

I believe you are right in saying that this will not allow you to perform to your full potential, and you are right to ask for a more humane schedule.