They already told you this in the discussion you posted but 'we are three' is not idiomatic English. You want to say that because you would say 'somos tres' in Spanish. It is understandable, of course but native English speakers don't say it like that. Yes, 'there are three of us' is the correct way. In speech, people would probably say 'there's three of us.' The reason? In speech we make great use of contractions, and "there's" rolls off the tongue much more easily than "there're", which is more awkward to say. If there are only two of you? It's the same. There are (only) two of us.

"We are three" is understandable but it sounds very formal and maybe even from an older dialect of english. "There are three of us" is good and sounds natural :)

I think when you say we are three, is true. because 'there are three of us' is like 'there are three of the books' The word 'us' is not an object. my English is not good, so maybe my opinion was false.