If you want to change the commit messages, the order or the number of commits, use:
$ git rebase -i HEAD~3
where HEAD can also be any other branch you are on and you can work on any number of commits, not just 3. You can delete commits, merge them together with "squash" or change their order. If something goes wrong, use
$ git rebase -i --abort
Note that this will change all the commit-ids in the moved commits. These are because the commit-id also takes the commit's history in to account and the same change in a different place is considered a different commit by git. Rebasing shared changes can make combining those changes down the track difficult - you will not normally want to rebase any changes which has been incorporated in to somebody else's or a shared repository.