使用此正则表达式:

.{3}$

如果您想避免在末尾留空格并可以使用捕获组(您没有精确说明语言或正则表达式的味道)，请使用

If you want to avoid spaces at end and can use capturing groups (you didn't precise the language or regex flavour), use

(.{3})\s*$

但是请注意，没有明显的理由在这里使用正则表达式而不是对字符串进行切片.

But note that there's no obvious reason to use a regex here instead of slicing the string.