I’d like to confirm what you mean by “domain B is also configured like domain A (the answer you suggested) in another office 365 account”. Do you mean that you have verified domain B in another Office 365 tenant?
If my understanding is correct, it is not feasible to add domain B into your account. Because one domain can only be verified in one Office 365 tenant.
On the other hand, if you already have an existing email account in domain B (such as user@domaiB.com ) and you want to receive the incoming emails of user@domainB.com via your Office 365 account, there is a workaround may meet your request. It is to add user@domainB.com into your Office 365 account as a connected account.
Please perform the following steps:
- Login to Outlook Web App via http://mail.office365.com/ .
- Click the gear icon->Options.
- Click connected accounts under account-> + ->then add your account within domain B.
In this way, you can receive the emails which are sent to user@domainB.com. And you can also send emails on behalf of user@domainB.com.
For domain C, please refer to the workaround above.
If there is any misunderstanding, please feel free to correct me.
Thanks,Colin