There is common trick in designing reliable systems out of unreliable components: replicating the unreliable component 3 times. When one of the three behaves differently from the other two, it's declared faulty and replaced. This wouldn't work with 2 copies as it wouldn't be possible to say which copy is faulty. So roughly speaking, for an outsider, the HN website appears as a single entity, but under the hood it's 3 replicas. This is an analogy to a proton that consist of 3 quarks.