Triples can have their own fun failure modes. Suppose a surge or the like fries two chips at once. It's rather rare, but it happens from time to time. The instructions come out and now you have three chips with different results. Which ones are bad and which ones are the good? More importantly, how do you recover without taking the entire system down. Redundancy in this way means you have the best chance of catching problems, and also recovering from those problems when things do go cactus.