What happens when one out of three NSs are down?

[Gary Wallis]

DNS experts:

What really happens in the real world when 1 out of three authoritative 
NSs are down for 30 minutes due to a datacenter outage?

For example, we have 3 NSs:

ns1.someisp.net 12.23.34.45
ns2.someisp.net 23.34.45.56
ns3.someisp.net 34.45.56.67

All in different datacenters.
All are authoritative for a given zone.
All have the same zone data and SOA serial number for the zone.

Where the datacenter handling ns3 broke routing (mistake in new router 
configuration) for 34.45.56.0/24 and ns3 is no longer reachable.

I think I have a grasp on the basic theory here, but in practice, the 
unreachable ns3 nameserver creates problems for a small group of 
customers trying to reach web sites with zones hosted by these three 
authoritative NSs.

Will round robin glue NS records help?

Can quick or automated changes at the registrar of the NS3 IP help? For 
example to change to a hot spare in some other datacenter? In this case 
would the running NSs have to have the changed NS A record also match?

Any comments and best practice solution info very welcome.

Thank you,
Gary Wallis