Amend (reset) the author of multiple commits at once

git rebase -i <COMMIT_SHA> -x "git commit --amend --reset-author -CHEAD"

For example:

git rebase -i HEAD~3 -x "git commit --amend --reset-author -CHEAD"

See https://stackoverflow.com/questions/4981126/how-to-amend-several-commits-in-git-to-change-author for details about how to preserve the original timestamps or about using --no-edit.