I've got a tenant with 3 domain names in it, which I'm the global admin for. I already had a shared mailbox setup for email@example.com. When I went to setup a shared mailbox for firstname.lastname@example.org, it does not seem to let me. Is this expected behavior and is there another way I should be doing this to make it possible? Thanks.
I wouldn't call it the expected behavior, but it's known. The reason is the alias part ("sales" in your case) and you can work around it by creating the shared mailbox via PowerShell and providing all the relevant values. Or, why dont you simply add this as additional alias on the already existing mailbox, do you really need 3 different ones?