The Ruckus ZD Smart Redundancy feature supports just two ZoneDirectors.
Do your ZDs have enough AP licenses for all three sites' APs?
You could define SSIDs for use at each site, then define AP-Group for each site, servicing only the SSIDs for that site.
The Standby ZD would have the same/synchronized configuration, and would take over in event Active ZD goes offline.
If APs are configured with Limited ZD Discovery (Configure->Access Points->AP Policies), so they know both ZD IP addresses,
they will fail over in the event of ZD offline / change in Active/Standby state.
Smart Redundancy is based on Standby ZD sending heartbeats every 6s and Active sending replies.
If no reply is received after 15s, the Standby will make a decision if it is disconnected with its peer, and
change state to Active. So you need less than 15s round trip on your VPN connections if ZDs are at
different sites. It may be wise to locate them both at your main site.
Yes, DNS is a viable method of pointing new APs to your ZoneDirectors. When doing so, both ZDs should have
separate records in the DNS server.
Primary ZD: 192.168.20.1
Secondary ZD: 192.168.20.2
zonedirector1.domain.local -> 192.168.20.1
zonedirector2.domain.local -> 192.168.20.2
A Management IP address / its DNS name cannot be used for AP's ZD discovery.