почему - more general, "why?" зачем - is used to know the ultimate aim - "for what purpose?" отчего - is used to know the origin (reason) of something - "because what?" https://www.italki.com/question/225626 https://www.youtube.com/watch?v=BRsEIfYEIeU

почему? - why (原因, 理由) зачем? - why (目的, 目标) отчего (we don't use it that often) = почему