Yes — if all the Business Central tenants (X, Y, Z) reside in the same Microsoft Entra tenant (Azure AD), then one paid full user license is sufficient for User A. The license is tied to the Microsoft Entra ID user, not to each Business Central tenant.
User A will be able to switch between those tenants without requiring multiple licenses, as long as they are all under the same Entra tenant.
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Nimsara