1. 三角(形);三角形物体
    A triangle is an object, arrangement, or flat shape with three straight sides and three angles.

    e.g. This design is in pastel colours with three rectangles and three triangles…
           这一设计由淡色的 3 个长方形和 3 个三角形构成。
    e.g. Its outline roughly forms an equilateral triangle.
           它的轮廓大致形成一个等边三角形。

2. 三角铁(一种打击乐器)
    The triangle is a musical instrument that consists of a piece of metal shaped like a triangle. You play it by hitting it with a short metal bar.

3. 三角关系
    If you describe a group of three people as a triangle, you mean that they are all connected with each other in a particular situation, but often have different interests.

    e.g. She plays a French woman in a love triangle with Jonathan Pryce and Christopher Walken.
           她扮演一位与乔纳森·普赖斯和克里斯托弗·沃肯有三角恋关系的法国女子。
    e.g. …the classic triangle of husband, wife and mistress.
           丈夫、妻子与情妇的典型三角关系