Hi Miranda,
Apologies for the late reply.
You can use the 'Course set due messages' to inform Learners when they must complete course sets. They can be set by using the 'minimum time required' setting in the course set.
The logic you could use would be something like:
CS1 (required 1 week) THEN CS2 (required 1 week) THEN CS3 (optional).
Hope this is useful.
cheers,
George.