Seperate volumes for tempdb, data, and backups on SQL Servers in VMs
I like to separate them even if they are pointing to the same drives for disk space reasons. If I run out of space on my backup drive/mount point, it's not going to break the database (not right away at least). 

This is how  I usually separate things at a minimum:

Data files for user databases
Log files for user databases
Data file(s) for tempdb
Log file for tempdb
Data and log files for system databases, except tempdb
Backups (though I prefer to use a network share instead)  

Each of the above would be a separate drive or separate mount point. Depending on the system, I might have separate devices for each database's data files.

But my systems have always used a SAN rather than local disks. If you had the choice between 3 drives or 5 drives, I would just ensure backups are separate and tempdb is separate. But again only for disk space reasons.