Never three exams in one day. In Mod 1, it's two on one day and the remaining exam on another day. The most information heavy exam always seemed to be by itself. I found the schedule to be totally doable even though it had been multiple years since I had taken two exams on one day.
In Mod 2, it's only one exam at a time, but they're definitely longer than the Mod 1 exams.
On a side note, the student portal:
http://www.med.upenn.edu/student/
is a great source of info in general. Some of the links require a Pennkey, which you'll get over the summer. But most of it should be available to you guys.