Three years后面系动词用单数还是复数？

单数，不管是一年还是一百年，都用单数。
如：
Three years is a long time.
三年是 一段很长的时间。

一般看成单数形式,表示一段时间,比如:
Three years is a long time.
有时候为了强调数量也把它看成复数形式,比如:
Three years were taken by him to do that.

three years is a long time
three years have passed since we met last time.
如果看作整体用单数,如果单指时间复数.