It is not necessary to get ALL 3 stars in one run, you can customise your vehicles for speed or durability and get the stars on separate runs.
Free Tokens
You can receive 3 free 'Hint' tokens by clicking on the hat and wrench icon and choosing to 'Like' Bad Piggies on Facebook.
Survey the Level
It is always worth taking the time to survey the level completely before you start constructing a vehicle to collect the stars as they can sometimes be situated in devious places, like the opposite way of the goal for example.
Various Game Hints
Choose for Longer vehicles, they are more stable then other smaller cars.
A number of objects like bottle rockets can be rotated when you open the option "Vehicle Editors" very handy.
Remember the fact that it is possible to modify the way vehicle behave in the game. So moving the objects in and around your fly machine, so try to manipulating fly abilities by shifting weight.
Make use of TNT if you'd like to shoot a piggy very far however be extremely careful.
Use the Umbrellas in order to reduce speed and to land softly on the ground.
There is no reason to smash all 3 targets at once. As an alternative, you can create vehicles updated with higher speed and better durability and get the goals on different runs.
Make use of both fingers so that you can can use zoom capability for that level, and figure out the complete situation at once.
Vehicles
When choosing a vehicle it is better to opt for the longer onese as they tend to be more stable then the smaller cars.
Zoom In and Out
To be able to see the entire scenario in on go use the pinch gesture to zoom in and out of the level. Use this useful feature to your advantage to make things easier for yourself.