Directly, they don't. However, don't forget about your homework just yet. The course material is very cumulative. You will find that it is much easier to keep up with the course if you do the reading and practice solving homework problems as we go along. Last year people who solved their homeworks did significantly better on the exams.
Exams focus on the material covered since the last test.
However, when writing exam questions I am assuming that you should know all the covered material. No, you are required to take any two exams out of three. Of course, you get more shots at a higher grade when you take all three. There are no makeups. All exams are given only once.