If you cannot commit to this, you should notify the editor at once and return both the paper and reviewing package. Editors understand that you may not have the time to do a thorough review, but are unforgiving if you commit to a review and then do a poor job. Also, good reviewing does not go unnoticed by editors, and can establish a useful reputation for you over time. Authors also have a responsibility to reviewers. If you submit premature work, not only will you annoy your colleagues who take the time to referee, but you will gain an unsavory reputation over time.
Researchers work in rather small communities and reputations spread.
Fill out the refereeing form right after the third reading, while things are still fresh in memory. Unless disallowed, you can return the editor a copy of the paper with your markup, to be returned the author. Some reviewers make a photocopy of the paper and transcribe their comments clearly on the copy. You can write the editor a separate, confidential, note if you wish. Many refereeing forms have a specific area for confidential comments for the editor.