First, one AAD could sync multiple forests: Topologies for Azure AD Connect
If you want to keep those mailboxes using original email address(such @domain1.com, @domain2.com, @domain3.com), you need to add those domain name into Office 365. Otherwise, those mailboxes will use Exchange online default domain name(@keyman .onmicrosoft.com).
So, it will not an issue that each domain has a info mailbox, if they using different domain name.
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