Help with git system

Dear opensuse members.
I have started once month ago using git locally in my computer to keep different version of my files.
For tha I am using git add file to add a new file to my git system
and 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.

Could you please help me 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 git.R as git2.R

Best REgards