Fire the first Black bird into the jutting ice block of the second tower (don't worry if you don't hit it while you're flying in the sky, close to the ice block). When the bird is just above the lowest triangle of the last tower, trigger the detonation. Most of the right tower will fall, the middle tower should topple to the left, and a fair amount of the left tower should be destroyed as well. Use the next Black bird to explode all pigs remain. In the video, pigs are in the two left and middle structure so you need to land him between the two structures. Sometimes, there are pigs at the right structure so you need to land there to pop it. If there are pigs in the middle, it is better suited to be lobbed into the center. There are more ways to get 3 stars but this is the way that most people use for the first time.