他敲了三下，这是他的习惯。

As was his custom, he knocked three times.

《牛津词典》

我决不低三下四地乞求施舍。

I wouldn't demean myself by asking for charity.

《牛津词典》

在电话铃声响第三下前接听电话。

Answer all incoming phone calls before the third ring.

youdao

I'm not going to writegiven threemoles of hydrogen at one bar and three degrees, blah,bla I'm going to write it in a compact notation.

我不会写“给定1巴和多少度下,的三摩尔氢分子之类,我会把它写成一个紧凑的形式。

麻省理工公开课 - 热力学与动力学课程节选

Now it's not uncommon, by the way, for laboratory animals in these studies to triple their body weight.

顺便说下,在实验室,让动物体重变成普通的三倍,并不罕见

耶鲁公开课 - 关于食物的心理学、生物学和政治学课程节选

And in this case we have three choices here in terms of what's going to be in the middle, so we need to decide that first.

而这种情况下对于哪个原子,在中间我们有三种选择，这是我们首先要确定的。

麻省理工公开课 - 化学原理课程节选