You can do this fairly easily without git rebase.
If you want to write the new commit message from scratch, thus suffices:
git reset --soft HEAD~3 &&
git commit
If you want to start editing the new commit message with a concatenation of the existing commit messages (i.e. similar to what a pick/squash/squash/…/squash git rebase -i instruction list would start you with), then you need to extract those messages and pass them to git commit:
git reset --soft HEAD~3 &&
git commit --edit -m"$(git log --format=%B --reverse HEAD..HEAD@{1})"