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. I found that Rebel units are more nimble and specialized, while Empire units are less efficient so I always run out of time. Darth Vader is also much stronger than Luke Skywalker as can be observed in missions 20.
Campaign Echoes of the Force 10 (3 Stars)http://youtu.be/3PczNutK6aw
Campaign Echoes of the Force 11 (3 Stars)http://youtu.be/pbitXU72ETM
Campaign Echoes of the Force 12 (3 Stars)http://youtu.be/Nml1DvPV68s