github: Adding commits to existing pull request

Yeah - you are doing a lot more work than you need to. Just make an additional commit and then force push it. You'll see the original commit as well as then newly pushed one when you refresh github in your browser.

$ git commit -m "These changes are in response to PR comments"
$ git push -f origin HEAD

Since you're using GitHub's tools and just changing one file, you could also browse to the file on GitHub, select the proper branch from the upper left corner under the "tree:" dropdown (patch-3 in your case), and now choose "Edit this file". Now your changes will be committed to this branch and will show up in your pull request