In order to get three stars in Theme 12-11, you must first aim the Yellow Bird at the vertical stone on the top of the first fort. This should kill three pigs and destroy several blocks, which will probably give you a score of around 20520. Aim the next Yellow Bird at the wooden square at the fort closest to the slingshot. This will kill two pigs, which earns a score of around 43930. For the Green Bird, fling him all the way back, then send him crashing in through the last pig remaining's fort. At the end of the level, you should have two birds remaining (White Bird and a Yellow Bird), so you would have a score of around 77160.