the third one, if you add the comma without the when then you would have a comma splice (when you put a comma between 2 independent clauses without a conjunction) adding the when before Cynthia makes the first part a dependent variable and makes it okay to add the comma!
Hope this helps! :)
It would cause the reader to think that too. and it could confuse the reader if the assumption is contradicted later on in the story.
hope i helped!
