Fabrikam identifies the following technical requirements:
When users leave the company, remove their licenses and ensure that their mailbox is accessible to Admin1 and Admin2.
If you remove the license from User3, the mailbox will be deleted after 30 days. Converting the mailbox to a shared mailbox will ensure that the mailbox is not deleted. You would still need to give Admin1 and Admin2 permissions to access the mailbox.
Reference:
https://docs.microsoft.com/en-us/microsoft-365/admin/email/convert-user-mailbox-to-shared-mailbox?view=o365-worldwide