You don’t need to create a storage partition if you don’t want to. You could theoretically reduce that partition table from the diagram to three partitions - one for each distro. It’s up to you, really.
The problem that I see with that is organisation and security:
you’ll likely use one distro more than the other two. That distro’s partition will get full of personal files faster. Eventually you’ll need to juggle files to the other two partitions.
you’ll need to remember which system you were using in order to remember where your files are. And since you’ll be juggling files back and forth, you’ll reach a point where you need to search three directories to find a file.
if your personal files are spread across multiple partitions, you’ll likely need to mount all of them in all your systems. This means that you’ll need to mount Debian’s and RHEL’s partitions in Arch, Arch’s and Debian’s in RHEL, etc. It’s generally not a good idea to mount partitions with system files unnecessarily.