Dirsync - does it have to be done at forest level?

Hi,

Scenario:

Single Forest
3 Domains (DomainA, DomainB, DomainC)

Each domain has a separate Azure Tenant, the key is not to have user "bleed" between tenants thus only users in DomainA are in AzureTenantA, users in DomanB in AzureTenantB etc.  As I understand it the only way to achieve this is to install a DirSync server per domain but at Forest level and then apply filters to stop the sync'ing of the all the users within the entire Forest into the Azure tenants.

Which brings me to the question in the title of this thread, does DirSync have to be done at the forest level?

Cheers
Rob