Enabling override on the primary FortiGate (optional)
Before adding the third FortiGate to the cluster, enable override on the primary FortiGate. In most cases this step would not be necessary but it is a best practice because enabling override makes sure the configuration of the primary FortiGate is not overwritten by the configuration of the new backup FortiGate.
To enable override, log into the primary FortiGate CLI and enter this command:
config system ha
set override enable