Translation:The third, fourth, and fifth baskets are pretty.
I am not great with grammar so hopefully someone can answer more clearly at another time. As I see it, you are denoting which baskets are beautiful out of a group, thus creating a separate group: the three baskets you think are beautiful. So instead of it being: "The third basket is pretty. The fourth basket is pretty. The fifth basket is pretty." ... It's more like: "This group of baskets is beautiful." I hope this helps somewhat. :)
What sounds good in English doesn't always sound good in Esperanto and vice versa. 'Mi lavas al mi la manojn' is good Esperanto, but ' I'm washing the hands to myself' is surely not good English. However 'Mi lavas miajn manojn' is also correct Esperanto. Do you want to know if 'La tria, kvara, kaj kvina korbo estas bela' means the same thing and is correct, or wether the English translation provided by Duolingo is correct?
I have no idea about this question. Maybe Duolingo made a mistake on this question in accepting "beautiful" too. Bela means both because I don't think there is a different word for pretty. Another guess would be that Duolingo opted out using "beautiful" to make learners aware that bela can be used to mean pretty. My last guess would be that the baskets aren't beautiful, just pretty :p