From what I understand so far, his question actually presumes a positive answer. The negative presumption phrasing (which would translate to what you wrote in Latin) would be "There aren't three women, are there?" — or, in duolingo's preferred phrasing, "surely there are not three women?"
So I share the same question — I would expect his English translation (which I also wrote and got wrong) to be accepted, and if I'm wrong, I'd like to understand why.
It is because duo and tres decline (change based on case) differently.
duo is used for masculine and neuter nouns. duae for feminine nouns. It follows first/second declension adjectives except for the dative and ablative.
tres is the same when used with masculine and feminine nouns and becomes tria for neuter nouns. It delines like a third declension adjective in the plural.