"I also have a hard time believing that any one cloud virtual server has many 9s uptime, so 3 together isn't likely to get you to Akamai's level."
Actually even if the servers are only 98% available (7 days of downtime per year!) then they provide 99.9992% availability, aka "five 9s": 1-((1-.98)^3) = 0.999992
Of course this statistical result assumes downtimes are random and uncorrelated. As you say the risks is that a common bug or vulnerability affects all 3 of them at the same time. Hopefully this risk is mitigated because my software stack is extremely simple (a web server serving static files + ~400 lines of Python code) and I will not be performing software updates on all 3 servers at the same time.
"or your DNS provider has a problem"
I do not use a provider, but run 3 parallel authoritative DNS servers on these 3 servers :)
Actually even if the servers are only 98% available (7 days of downtime per year!) then they provide 99.9992% availability, aka "five 9s": 1-((1-.98)^3) = 0.999992
Of course this statistical result assumes downtimes are random and uncorrelated. As you say the risks is that a common bug or vulnerability affects all 3 of them at the same time. Hopefully this risk is mitigated because my software stack is extremely simple (a web server serving static files + ~400 lines of Python code) and I will not be performing software updates on all 3 servers at the same time.
"or your DNS provider has a problem"
I do not use a provider, but run 3 parallel authoritative DNS servers on these 3 servers :)