"我三年前就认识他."用英语如何说?

用一般过去时和现在完成时都可以,看在什么情况下。
如果对方的重点时你是什么时候认识他的，用一般过去式
I knew him three years ago.
如果对方重点在问你认识他多久了，用现在完成时。
I have known him for three years。

I could know him three years ago .或I knew him three years ago.
前面的同志们，要记得“三年前”是一般过去时！

I had known him three years ago

用一般过去时
I knew him three years ago.
用过去完成时
I had known him three year ago.

以上两种都可以。

I knew him three years ago.
一般过去时。
前面的朋友，有点公德心，不要误导别人啊！

My three years ago know him