3:45的英文是a quarter to four.

quarter：n.四等份之一;(正点之前或之后的)15分钟，一刻钟;三个月时间;季度;季。

four：n.四;四个;第四四的;四个一组之物。

相关介绍

It was a quarter to four and the last class began.

这是一个季度的四和最后一节课开始。

We should arrive at a quarter to four.

我们应该差一刻四点到。

You say you've got to get there at seven but it's only a quarter to four now.

你说你必须七点钟赶到那儿，可是现在才四点差一刻。

A quarter of all the people surveyed were don't-knows.

参加问卷调查的所有人中有四分之一没有表示明确意见。

August registrations have gone ballistic, accounting now for a quarter of the annual total.

8月份的注册人数突然暴涨，现在已经达到了年注册量的四分之一。