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2. Just two failures. To show why three aren't possible, suppose that primary function A, backup 1 and backup 3 all failed. Then function A would have no device to do it. On the other hand, two failures cause no problems.
Here is why: If two backups fail, then all functions continue. If two primaries fail, then the warm-up shows that all functions are covered. If one primary and one backup fail, then the other backup that covers that primary will take over.