[Feature] Minimum Subnets: 3

[todd-dsm]

What feature/behavior/change do you want?

I'd like to suggest the minimum number of subnets be changed from 2 -> 3; docs.

Why do you want this feature?

The SRE Handbook describes Capacity Planning like:

Provision to handle a simultaneous planned and unplanned outage, without making the user experience unacceptable; this results in an "N + 2” configuration, where peak traffic can be handled by N instances (possibly in degraded mode) while the largest 2 instances are unavailable.

While they are specifically describing nodes, the concept could be expanded to mean any fundamental resource. Given a stellar (but imperfect) history of AWS' uptime, it seems that three (3) would be an improved number to suit the foundational concept of a distributed system.

In this way, we could be slightly better off knowing that we could withstand an unplanned outage in the middle of a planned outage.

What say the council?