Having just learned tio and tiu I notice they are not in this sentence, therefore, to me the translation should be "In the house are three humans." Which is more a germanic turn of phrase than an English everyday one but can be used as a descriptive sentence in a book. Why is it not showing "There" in Esperanto?
While many English-speakers might contract/slur words and things like "there're" or "shouldn't've" might be heard often (or at least it is where I live, in the Midwest) technically it's not proper English or a "correct" contraction, which is why I really doubt that Duolingo would accept it as an answer even if you reported it.
Especially when typing instead of speaking- I can't recall seeing "there're" typed out anywhere else before.