To create a new branch (locally):
With the commit hash (or part of it)
git checkout -b new_branch 6e559cb
or to go back 4 commits from HEAD
git checkout -b new_branch HEAD~4
Once your new branch is created (locally), you might want to replicate this change on a remote of the same name: How can I push my changes to a remote branch
For discarding the last three commits, see Lunaryorn's answer below.
For moving your current branch HEAD to the specified commit without creating a new branch, see Arpiagar's answer below.