Dobrý vs Dobrou
What's the difference between dobrý and dobrou? Bonus: where does dobré fit in with those two?
They are all forms of the same word. See here https://www.duolingo.com/skill/cs/Masculine/tips-and-notes (particularly the adjective section, but the part about gender is helpful, too).
You might find the Tips & Notes for the Adjectives skill helpful, too.
and, not to forget, "dobrou" is accusative (the feminine nominative is "dobrá".