Nobody can really learn a language in 3 days, but Duolingo user julesmGGF learned quite a bit of Esperanto from zooming through tree in that time! Jules plans to continue studying Esperanto to actually master the language but we're impressed with the progress made in such a short amount of time.
Also see Esperanto in 6 Days with Duolingo!
https://www.youtube.com/watch?v=ClKNspptp-o