Why are Zeus poseidon and hades called the big three in Greek mythology?

They aren’t. That’s a modern invention, as ancient Greeks did not have this concept. Hades was one of the three brothers who did divide the universe with Zeus and Poseidon, but he was not one of a “big three” gods. He was not widely worshipped and was mostly ignored. If anything, Apollo would have been the closest to a third member with Zeus and Poseidon.