No because it would be "Sie essen fisch" somethings just have to be taught in a classroom. This is because when you conjugate "essen" to eat it would be "essen" because "they" is Sie with the "en" ending. Its confusing but gramer isnt taught in this app only the most basic vocabulary. Hoped this helped
They=sie. She=sie. Formal you = Sie. You only capitalize "sie" when it's the first word in a sentence or when using formal you. Duolingo hasn't mentioned formal speach yet in these lessons. So, Isst sie Brot? = is she eating bread? Essen sie Brot? = are they eating bread?
Formal you is used when you are talking to a stranger or a person senior to you (in age or professional status) or a person you are serving (a waiter would use formal you with an adult diner, a nurse would use formal you with an adult patient)--or if you're a child talking to an adult. Formal you takes the plural verb. Essen Sie Brot? = are you eating bread? (And the plural formal you is likewise capitalized-- Ihr.) I agree that Duolingo isn't really a good way to learn a language from scratch because sometimes you just need to know the grammar to understand why you say something in a way that is very different from your native tongue. Other times, though, the answer is "that's just the way it is; memorize it." (For someone brushing up on a language studied years ago, though, Duolingo is wonderful!)
When 'sie' is used for 'she', the verbs end with the letter 't' example - "Sie trinkt wasser" which means "She is drinking water" but when 'sie' is used for they, the verbs end with 'en' example - "Sie trinken wasser" which means "They are drinking water". I hope this helped you