Yes, triple redundancy is nice, but then the problem becomes how to make the code that checks the code also tipple redundant and checked without creating another layer to deal with.
Most software that does this simply tries to make the test code as small and reliable as possible. But it's an interesting challenge to make the test code somehow self checking with the same properties.
Well, physically, you really need three independent machines. Which means three copies of the file, three processes, three verifications, and so on, and a way for those processes to communicate with each other and "vote". Then you run into the problem of definition - is a cluster a quine? Note that most of the "primordial bit flip" problems go away if you make N >> 3 and design each machine to "fail fast".
