Approach to sync multiple forest AD users in Azure AD

Hi,

We are developing Intranet built on SharePoint 2013 over Office 365 E3. Now, there are many users in multiple AD forest at different geographical locations.

Now they want all users should have access on Intranet (SSO will be preferred). So they planned to sync multiple forest users to Azure AD.

We are suggesting approach using “AAD Sync tool/AAD Connect tool/FIM”.

We are looking for approach to sync multiple forest AD users in Azure AD.

Any information, challenges and approach about this would be greatly appreciated. Thanks!