Hi westhk,
I would like to confirm whether you want to forward emails to multiple users or want A forward emails to B and then B forward it to C.
If you mean the former, you can refer to the steps below:
1. Login to Outlook Web App (http://mail.office365.com/).
2. Click the gear icon on the right top corner->Options.
3. Navigate to organize email->inbox rules->create a forwarding rule as the picture shows:
(More options)
(You can add up to 10 recipients here)
If you mean the latter, I regret that it is not feasible because of the Exchange Online limitation:
Please feel free to let me know if I have any misunderstanding.
Thanks,
Chris Ni