I think the main reason is that there are no equations that involve the third derivative explicitly.

我想主要原因是因为还没有等式,明显涉及三阶导数

耶鲁公开课 - 基础物理课程节选

Well-being offerings are of three main types.

健康祭品主要有三种类型。

耶鲁公开课 - 旧约导论课程节选

So there are three main geographical subdivisions.

有三种主要的地理形态分类。

耶鲁公开课 - 旧约导论课程节选