Triangle
noun, の adjective
When you have something with three angles, what shape is that? It's a triangle.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
そのチーズは三角でした。
The cheese was triangular.
パイナップルは小さく三角に切って下さい。
Cut the pineapple into small triangles.
三角と四角の二タイプのタイルがあります。
There are two types of tiles, triangular and square.