All three would work provided the gender of the word matches the subject talked about. "Niemand anderes" seems to be the catch-all case that can cover all genders. The indefinite pronoun "andere" and its various forms can be contracted. So "anderes" and "andres" are the same. However, in some dialects (like Austrian) you can also say "niemand anders" (note that the last 'e' has been eliminated as opposed to the first one.)
I gave 'nobody else has answers' as the answer to this. Apparently that's wrong, but I don't know why, as this seems like a pretty good translation to me. The correct answer that Duolingo gives is 'nobody else has returns'. As a native English speaker, I've literally no idea what that means. Surely some mistake here?
They're not the same at all! "Nobody" would mean nobody at all, but "Nobody else" is distinguishing from another group previously mentioned-- this group may or may not have answers.
So I could say "John figured out the problems, but nobody else has answers"; this means that no one other than John has answers.
I'm using the smartphone app and with antworten being yellow and on the second line all by itself, i didn't notice it the first time i went through this section. It's such a minir problem...but i thought i would report it....and since the app doesn't have the 'report' button that i can see....I'm just gonna leave the report here