I currently have three modified files in my working directory. However I want one of them to be reset to the HEAD status.
In SVN I'd use
svn revert <filename>
svn update <filename>
git reset --hard
You can use the following command:
git checkout HEAD -- my-file.txt
... which will update both the working copy of my-file.txt and its state in the index with that from HEAD.
Thanks to VonC who points out this answer, which explains why you might need the -- in this command.