When backing up many Macs to one hard drive, must the hard drive be partitioned for each Mac? I'm getting a 5TB drive which I plan on using for backing up all my media, and my OS/config for my machines. The backup tool is Carbon Copy Cloner. I currently have two Macs but may get a third one and I'm wondering: 

Since the laptops have different OSs and configurations, do I have to partition my 5TB drive for each of these like this:

╔════════════════════════╗
║ 500GB - Mac #1 Backup  ║
║════════════════════════║
║ 500GB - Mac #3 Backup  ║
║════════════════════════║
║ 500GB - Mac #2 Backup  ║
║════════════════════════║
║                        ║
║                        ║
║ 3500GB - All Media     ║
║ (movies, music, pics)  ║
║                        ║
║                        ║
║                        ║
╚════════════════════════╝


...Or, is it safe to put all three machine backups onto the same partition like this:    

╔════════════════════════╗
║                        ║
║                        ║
║ 1500GB - 3 Mac Backups ║
║                        ║
║                        ║
║════════════════════════║
║                        ║
║                        ║
║ 3500GB - All Media     ║
║ (movies, music, pics)  ║
║                        ║
║                        ║
║                        ║
╚════════════════════════╝


I'm suspicious that the former is necessary for the purposes of restoring from a backup so the partitions are "bootable," but I don't understand why this may be needed. If possible, I'd like to use the latter partitioning configuration and just use folders to differentiate between the various Mac backups. Then if something goes wrong, I have the files that I can just drag into place on the machine that I need to restore.

Note that I am using redundancy with two other 1TB backups but I don't want to rely on those for restoration, only for storing duplicates of my media.
If you want the partitions to be bootable, yes you will need a separate partition for each mac you're backing up. The only way to have multiple backups in the same partition is to separate them out into other directories, which is not something most (all?) computers will be able to handle when trying to boot.

If you use Time Machine to do your backups, you could back them all up to the same partition. Then while the partition is not bootable, when you boot into recovery on the given mac, you can point at the drive and Time Machine will distinguish between the various backups.

The final, most convoluted option could be to use Read/Write disk images, those could be mounted, and backed up to, and then if a system failed, you could directly image the disk image to the new drive. This doesn't buy you a whole lot over backing up to separate folders and just copying the contents of that folder over to a new drive, so I wouldn't really bother with it.