[Kurtz79]

It was mostly a side comment, but it seemed appropriate in this context.

For such an important system, I would think that a single instance would be too little (single point of failure: even in less mission critical system it wouldn't be allowed), while using two you would not be able to resolve a reading conflict between them (which is the point of the quote), so three is probably a reasonable number.

Even if applied to design and not to software programming, the concept is still sound. The point was what the quote meant, not its source.

[weaksauce]

Using two would have been strictly better a design than just one. Obviously three is better but two disagreeing and shutting down mcas would be much, much preferable than relying on one to the grave.

[vkou]

Preferable to the passengers, but not to Boeing, because that little warning light would warrant retraining and re-certification of pilots.

It's why they didn't do it.