I got 3 stars on all levels ages ago...still play every now and again to try to better my scores...

That level...
aim for the top TNT...
it will explode and fire the small rock at the big rock to the right...
that will roll down and land on the next far right TNT...
that will explode and fire the small rock off to the left....
that will (hopefully) hit the TNT in the middle at the bottom....
that will explode and fire the small rock to the right...
that will hit the large rock on the far right, sending it rolling down, killing many piggies....
If you have any piggies left, fire ya next bird at the large rock on the left at the bottom...that will roll down and kill the remaining piggies....

It is easy to kill all piggies in 2 shots....

The trick/skill is in killing ALL piggies in just 1 shot!
It can be done, I have done it

Good luck