I have 3 iPhones that I want to back up separately in order to keep the business and personal data entirely separate. Can I do this in a single iCloud account or do I need to setup separate accounts for each device?

How do I remove an iphone from my current account and set it up as it own account

You can do this on a single iCloud account as long as all 3 iPhones are using and signed in with the same Apple ID.

OR

If you have the iPhones on separate Apple IDs then you can use the Family Sharing feature and share one iCloud account.

OR

You can just have separate iCloud accounts for each iPhone.

In all of the examples above, each iPhone will have its own backup file.

Axel F.