Think of an outline with three points sticking out: a folded scarf, an old sailor's hat, a slice of sandwich. Wherever you look at it from, there are three corners and no more. Once that outline is in your head, the word can also be laid over a situation — three sides pulling at the same thing, each one a corner.
Two branches
看得見的三角形狀
這一支就是畫面本身，沒有經過任何轉換：東西真的有三個角。
adjective · B1
Having three corners, so that the shape is a triangle. It is used about things you can see and hold — a hat, a scarf, a piece of cake, a sail — and, in writing about plants, about a stem that has three ridges running along its length.
三角形的
三方在搶的局面
把三個角從形狀搬到局勢上：每一個角就是一個對手。這是一種理解方式，不是在說歷史上真的先有形狀才有比賽的用法。
adjective · B2
In the expressions a three-cornered contest and a three-cornered fight: a competition, election or struggle with three people or groups in it, instead of the usual two.
三方對決