since three years ago用什么时态

since three years ago用现在完成时。

现在完成时态构成：

主语+have/has +过去分词+其它。

现在完成时态的标志词常用的有：

1. for +时间段

I have learned English for three years.我已经学习英语3年了。

2. since+一般过去时的句子

I have lived here since I moved here.

自从我搬到这里，我就住在这里。

3. since +时间段

She has lived here since 3 years ago.

自从3年前，她就住在这里。

4. so far到目前为止

We have finished the task so far.

到目前为止我们已经完成了这个任务。

5. in /over the last few years在过去的几年

We have changed a lot in the last few yesrs.在过去的几年我们改变很多。

since three years ago用什么时态

since three years ago所在句子的时态要用现在完成时态。如：

They have planted many trees since three years ago.

since可作介词用，和表示过去某一时刻的时间状语连用时，所在句子要用现在完成时。

since也可作连词用，意思是“自……以来”，引导时间状语从句，如果从句用的是一般过去时，主句要用现在完成时。Great changes have taken place since I came here.