How to cite a course overview note?
Since the course overview note is credited to Professor Z, that's who I'd cite. But you could include all the authors in the citation itself if you liked, for example:


Professor Z (2013) Title of the Course Notes. Distributed as part of the course materials for DEPT304 Course Title in Fall 2013, a course developed by Professors X and Y at the University of Hishouse. Available online at http://example.org/.


Of course, it would be ideal if you could find a more widely available source for this information; it might be hard for your readers to track down this one course overview note, but if you could find the same information in a textbook or a review article and cite that instead, I think that would make their job a lot easier!