On Premise Appian High Availability (18.04)

When setting up Appian to be HA on premise following the instructions in: https://docs.appian.com/suite/help/18.4/How_to_Set_Up_High_Availability_Installations.html

how fault resistant is the setup.

If Appian is running on 3 servers as recommended can 2 of those 3 fail and Appian continue to operate or do there need to be at least two instances running so that consensus can be reached for writes?

Does it need to have a quorum to establish a primary (involving at least 2 instances)?

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