The key to passing this stage with three star rating is to time the bomb. Wait for the bomb carrying pig for 1 second before you take your shot. You will use the bomb to knock down the buildings and kill all the pigs.
First Shot Result
The bomb kills the pig on the left, while you kill the pigs on the right. You will then be rewarded with the three star rating. This stage may take you a couple of shots before you can get the perfect timing. Note that the bomb has to land on the gap shown in the picture.