I honestly was not sure about what was meant by this. Couldn't we also have said "Are you three?" because of the plural form of "sunt"? And the "-ne" doesn't indicate anything as far as I remember it from my Latin lessons in school, right? Would my answer be appropriate?
Remember, "sunt" is third-person plural, so "you" won't work here.
As for the '-ne' suffix, it just makes a statement into a neutral yes-no question as opposed to 'num' or 'nonne', where a 'no' or 'yes' (respectively) is expected. More here: