Here's the "How to 3 Star" from the Empire side. Empire seems initially stronger but I found it easier to win with Rebels during the later missions. Darth Vader is also much stronger than Luke Skywalker as can be observed in missions 20.
Campaign Echoes of the Force 22 (3* Stars)
half a sec late.http://youtu.be/Izibw_gWcoQ
Campaign Echoes of the Force 23 (2 Stars)http://youtu.be/8yXIXZAwG4U
Campaign Echoes of the Force 24 (3 Stars)http://youtu.be/8NgWhGBnMpc