Well, I don’t think упражнение is used in business contexts. Задача can translate to "objective"/"aim"/"task"... That is what you have to accomplish. Задание is similar, but it’s more like something that other people tell you to do; at school it’s a teacher, at an office it might be your boss.
Apparently because you're expected to translate that as «упражне́ние но́мер три».
Or perhaps simply «упражнение три»
We have seem both
шестой кабинет = "the sixth room", which Duo accepted but translated alternatively as "room six"
кабинет двадцать пять = "room twenty-five", which doesn't seem like it could be translated as "the twenty-fifth room"