Why turista and not turistak? Because there are three tourists, shouldn't the noun be plural? Is this a duolingo problem, or a Hungarian structural difference with English
Yes, this is a difference between English and Hungarian. In Hungarian, the noun stays singular after any quantifier, number or amount or whatever.
We do not use plural in this kind of case. Logics... There are 3 of them, everyone knows it and that they are more than 1 so you do not need to stress it again in the noun as an ending. That would sound pretty silly and odd.