Well the big 3 is more of a term for the 3 dominant players of this era; Federer, Nadal & Djokovic. If you're saying that he's one of the 3 best players at the moment I'd agree, I think there's an argument for him being the 2nd best player in fact.
Well, many considered thiem part of the big 3 contenders during this tournament and he is clearly the 3rd best player in the world right now. Murray was part of the "big 4" in 2008-2013, it doesn't mean dom is close to the other achievements just that he is a consistent dominate force in the top for a while. We will see how he follows it up.