I have some untranslated mangas that come in pairs of two, and each of them is labeled with 上 or 下 to denote which is the first volume and which is the second. I'm wondering if these labels are only used for pairs of two or can also be used for trilogies, maybe in the order of 上 - 中 - 下 perhaps, but I don't have access to library or bookstore for Japanese books to verify.
So, are the usage 上 and 下 as labels only limited to books/movies/etc that come in pairs of two?
p/s: On related note, in ダンゴ３兄弟, the eldest dango is 長男, the second is 次男 and the youngest is 三男. If they were actually ダンゴ二兄弟 instead, would they be labelled with 上 and 下 instead? I think I've heard "上の子" or "下の子" being said but I'm not sure if they are referring to the order of siblings or something else.