The students came into the classroom in twos and threes 一句中为何用twos and threes

三三两两的有好多堆?

三三两两的？

三三两两的
例句：
They sat in twos and threes on the trim barbered lawn.
他们三三两两地坐在修剪平整的草坪上。

The students are coming into the classroom by twos and threes.
学生们正三三两两地走进教室。

twos and threes意为三三两两，是习语
(详见“英语竞赛培优”中的原题）