I noticed the same thing as well. In English all three of those phrases mean basically the same thing as far as I am aware, so I think it should be accepted. Maybe it is because "I'm doing good" can have multiple meanings (doing well vs. doing good deeds). Its literal meaning might be the second one, but its use in common speech is now both.
This expression is technically ( it goes well for me, thanks.), but the English expression is "I am well, thanks." Indirect objects also use the Dative case in German and many indirect objects are translated as "to me" or "for me" using the prepositions "to" or "for" with whichever pronoun is involved. Some German verbs require Dative even though in English, they might not. https://www.thoughtco.com/frequently-used-german-dative-verbs-4071410
i translated i'm fine as mir geht's gut but was marked as wrong, though it is correct German, geht's is more colloquial, the "es" is often dropped and replaced with an apostrophe in the written language. please correct the wrong answer response for Mir geht's gut , though your originally taught mir geht es doesn't make Mir geht's wrong!
originally you said gut meant good, so how does it now mean fine??? English speakers are more likely to say good.