Bad Piggies When Pigs Fly Bonus Level 3-I (3-1) can be 3-starred in two steps. To obtain one box, go through the middle path with the fan in front and bellow behind. For the other, remove the back wheel and follow the top path. You will need to pop a balloon once you're up there.
If you have a different strategy for completing this level feel free to leave a comment below.