Connecting Multiple Office 365 Domains to Azure Active Directory

We have 3 Office 365 domains: myschool.edu, mail.myschool.edu, and my.gradschool.edu. All 3 Office 365 domains have corresponding AD user accounts on our local on premise AD. We would like to sync all the accounts from the 3 O365 domains to our myschool.edu Azure Active Directory. Is this possible and how would I go about doing this? 

Ultimately, we are trying to establish SSO or password sync for our new cloud based learning management system, Office 365, and local systems. Is there a best practice in pursing this? Thanks!