How to remove an old commit in Git : All:

I am pretty new in Git, I wonder say I have submitted several commits like:

1 -> 2 -> 3 -> 4

Could anyone show me the steps how to remove commit 3?

Say each commit I just append that order number to same file.

So for 
1: 
the file content is 1.

2: 
the file content is 12.

3: 
the file content is 123.

4: 
the file content is 1234.


Thanks