Registering devices with Intune for management and policy enforcement
Unfortunately no - a Microsoft 365 E3 Intune entitlement is user-based and is assigned to the named licensed user. It is not a transferable or floating license that can be used to cover a mobile device permanently assigned to a different, unlicensed person.
An E3 user license allows that specific licensed user to use Intune to manage their devices. If User A has Microsoft 365 E3 but a mobile device is permanently assigned to User B, User A's Intune entitlement should not be used to license User B's device simply by enrolling it with User A's account. The fact that the physical device itself can technically be enrolled does not make the licensing compliant.
For shared, user-less, or dedicated devices, such as shared phones or kiosk-type devices, Microsoft provides Intune device licensing. In that scenario, an Intune Plan 1 Device license can be assigned to the device rather than relying on a named user's Intune entitlement. For more, refer to Microsoft's licensing guidance: https://learn.microsoft.com/en-us/intune/fundamentals/licensing
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hth
Marcin