Last could work well, but it might be best to avoid the suggestion that ghost Luke will never teach her anything else again. I’d still be willing to try it though.
Do you know if he does actually say “third” at any point during the movie?
EDIT: Did another test. The inflection Mark does with the word “lesson” makes it so that it would only sound natural as the first part of the phrase. So it would have to be a word that makes sense after “lesson”.