If I have n commits, how can I branch from the n-3 commit?
I can see the hash of every commit.
You can create the branch via a hash:
git branch branchname <sha1-of-commit>
Or by using a symbolic reference:
git branch branchname HEAD~3
To checkout the branch when creating it, use
git checkout -b branchname <sha1-of-commit or HEAD~3>