To pass this stage, you cannot pull the slingshot all the way. You will have to pull it lightly enough to pass through the little gap between the hanging metal. Your goal is to bypass the little gap and trigger the bomb on the ground.
Note that although you can kill all the birds in one shot and obtain the three star mastery, it is very hard because getting to the bottom can be really tricky. You can also obtain the three star mastery as long as you finish the stage in two shots.
Power Activation Point
Activate your laser gun at the picture shown. Your goal is to break the chains and setting the floating platform free.
First Shot Result
Your flying pig will detonate the bomb on the bottom, killing the two birds. The floating platform gets set loose and kill the rest. You will then obtain the three star ranking. This stage is very tricky and require multiple tries to make it happen.