1- Open the game
2- Open the campaign
3- Open the level that you want win 3 stars
4- Click in "Done" without deploy any troops
5- Open PvP
6- Fight anyone that you sure you can get 3 stars
7- Open the campaign again
8 - Open the level you want again
9 - (here's the trick) press home button of your phone.
10 - END THE PROCESS IMMEDIATELY by double clicking the home button and swiping up on the Star Wars process
When you go back to the game you will see more campaign points even if you don't see that it gave you stars.