Hi @CostasD ,
In my opinion, there is not much difference between the two.
For shared mailboxes, you need to grant the user mailboxes full access permission of the shared mailbox to view its contents. Therefore, these user mailboxes need to be assigned licenses.
Creating a shared mailbox itself doesn't require any license.
Because you want company members to be able to see who is operating these three private mailboxes, please consider the following scenarios:
1. Convert a special-purpose mailbox to a shared mailbox.
2. Create user mailboxes and assign licenses for temporary employees.
3. Grant Full Access/Send As permissions to the shared mailbox to the desired user mailbox.
You can refer to this link to see what happens in the shared mailbox:
Manage mailbox auditing - Microsoft Purview (compliance) | Microsoft Learn
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