You can use two shots to earn the triple star mastery in this stage. Your first shot should aim for the space junk at the top of the level. Because of the laser shooting pig, you may have to try a couple of times before you can achieve the three stars.
Power Activation Point
The point of using your power is so that you can have more momentum to push the big metal piece.
First Shot Result
The lasers reflected off the metal piece, killing most of the pigs on the surface. This shot should kill all the pigs except the laser gun pig and the one next to it.
Aim straight at the space junk near you.
Second Power Activation Point
Activate the power to increase the area of your impact. Your goal is to push the metal junks into the planet to kill the rest of the pigs.
Second Shot Result
The triple mastery stars are earned when all the pigs are killed after you push the space junk.