I wonder how much of EPS 3 is going to be GL's own personal wants and how much he will put in "because the fan's want to see it." Like Vader emerging. I have talked to a number of fans around my area that feel that if Vader is placed in the film too early that it wouldn't be a true EPS 3. Somewhat like having EPS 2 taking place during the heat of the CLONE WARS. Each episode takes place over just a few days so the only way an episode fits into the big screen is one LARGE major event and its resolution takes place at the end.
EPS 4- Destruction of Death Star 1
EPS 5- Rebel Alliance escapes almost near complete destruction.
EPS 6- Destruction of Death Star 2
EPS 1- Trade Federation defeated or (from the perspective of the OT) Palpitine elected Chancelor. (AKA) Sidious' first phase of his ultimate plan.
EPS 2- Beginning of the Clone Wars/Sidious' second phase of his ultimate plan.
EPS 3- (Hopefully with the pattern of the previous 5) the birth of Vader and the Empire.