The Gen 3 Wall Connector manual explicitly states that each charger "must have its own branch circuit". You can feed a subpanel from a single breaker. but each Wall Connector should have it's own breaker in the subpanel.
I expect that this is done to provide fire protection because doing this eliminates any unprotected wire junctions. If you feed multiple Wall Connectors from a single breaker, then you will need to form junctions downstream of the breaker that are not monitored. Each Wall Connector has temperature sensors that will detect a faulty connection at the baseplate. But a wire junction - even inside the breaker panel - is unprotected and could lead to fire or severe wiring damage.
This is the same reason that the Gen 3 Wall connector is not to be used with an outlet. The Gen 3 Wall Connector does not have provisions for a temperature sensor in a power connector (the Gen 2 Wall Connector did). Without the temperature sensor, a faulty connection at the outlet can lead to undetected overtemp and fire.
