Remind me of the Ariane 5 maiden flight: redundant hardwares does not protect against unsanitized inputs. I don't know much about the design of the airbuses, but I think it's sensible to specify the last primary computer running in a different mode as the first two, simply because double hardware failure is very very unlikely. So it boils down to either software bug and or unsanitized inputs. Assuming that, the last computer should provide some basic but safe functions.