Following on from @mzimmerman if you are going to crosslist you need to do so predictively; its good news if you have an existing crosslist that your third course will be added to, as long as it hasn't already started.
If you start a new crosslist AFTER students have engaged, then Canvas will disassociate the students from their activity in the original course that has been left behind - it still exists but is more difficult to get to. So if data is important to you, review whats's coming up and make your crosslisting choices early.
I would expect that you might still have to maintain separate assignments for each of the three sections.
Further, Instructure designed courses on the basis that students were in the same course because they all access the same material (not always the case - we have courses which are co-taught at different levels with differentiated materials. Hence if you want to make CONTENT specific to one or more sections, you would need to add it as a discussion, activity or assignment and choose the relevant sections as the audience.
