Apple needs to be more clear about how AppleCare One works. AppleCare+ is linked to the device, as evidenced by a serial number. I have AppleCare+ for my iPhone and for two other devices that I bought which are used by family members. All 3 are covered by AppleCare+, and all 3 were purchased under my Apple account. However, the other two use different AppleIDs to log into iCloud, so they can sync their own contacts, photos, music, etc.
I called AppleCare to confirm all 3 devices would be covered by AppleCare One and was assured that they would. However, after switching to AppleCare One on one device, the others couldn't be added, because they use different AppleIDs for their content.
So AppleCare One is designed to cover 3 of the devices that an individual uses, like their iPhone, iPad and Apple Watch..... it is NOT designed to cover 3 devices in your household, some of which may be used by other family members.