"Are you a singer?"
énekes vagy? (informal) would suffice. Ön énekes? (formal)
If anyone else wondered why 'Te énekes?' doesn't work, it's because it would translate to 'you singer?', making you sound like a somewhat cultured caveman :')
The formal form works differently though, and 'Ön énekes vagy?' and 'Ön van énekes?' or 'Ön énekes van?' all make no sense; the only way for the for the formal to work in a sentence like this is to just have formal pronoun + noun + question mark
It is not 3rd person, but one of the polite forms of 2nd person. 2nd person singular may be te (informal), or maga or Ön (formal). I don't think Duolingo uses maga much if at all. The formal ones, although they are 2nd person pronouns, do use the 3rd person form of the verb, when there is one.