I am reminded of the warning that zonemaster gives about putting your domain name servers on a single AS, as is common practice for many larger providers. A lot of people do not want others to see this as a problem since a single AS is a convenient configuration for routing, but it has the downside of being a single point of failure.

Building redundant infrastructure that can withstand BGP and DNS configuration mistakes are not that simple but it can be done.

As the CPU/RAM resources to run an authoritative-only slave nameserver for a few domains are extremely minimal (mine run at a unix load of 0.01), it's a very wise idea to put your ns3 or something at a totally different service provider on another continent. It costs less than a cup of coffee per month.