19 and 20 get a little bit harder. Had to try a couple of times to get the 3 stars.
In 19, make sure to note when the speed bikes are dropped. They can often get bugged into shooting the wall while the storm troopers are already attacking it. If you are short on time, just remember, you get 3 stars when all the buildings are destroyed. Left over units (e.g. Dark Troopers) do not matter.
In 20, make sure Skywalker is sent in on a delay so his arrival is timed with when most of the storm troopers start shooting. Remember, he can only deflect blaster fire - his ability does not work on rockets or melee attacks.
At this rate, not sure I can 3 star the later levels. Good luck.
Campaign Echoes of the Force 19 (3 Stars)http://youtu.be/EeWMWcwb7P4
Campaign Echoes of the Force 20 (3 Stars)http://youtu.be/wwuMzYV2afw
Campaign Echoes of the Force 21 (3 Stars)http://youtu.be/70O3vD2JlA0