Software :: Restore As Current Git.R The Third Commit Out Of Four?

I have started once month ago using git locally in my computer to keep different version of my files.
For that I am using

Code:
git add file to add a new file to my git system
and
Code:
git commint
to commit changes.

Let's say that I am implementing a file called git.R and I have commited so far 4 times.

understand what is the best way

a) To check the differences between the changes I did to the file git.R
b) Restore as current git.R the third commit out of four I did
c) Restore the second commited git.R as git2.R