The best thing to do in this game is to just play through all the levels in the first two worlds without worrying about 3 stars. Once you finish The Lost City you'll get the Detonator which allows you to detonate the grenades at any time, even in mid air. This makes many levels so much easier. Once you have earned this and completed the first two worlds go back and replay the levels to earn 3 stars.
Even with the detonator some levels can still be tricky because the detonator may not help you if the enemies are buried underground or behind layers or obstacles. I found The Lost City the easier to 3 star all levels, followed by Desert Strike, but both are pretty easy.
Ultimately you should work towards both because you'll need to finish off 3 entire worlds to unlock the following: