I've been trying to get 3 stars on level 20 for a while and I can't figure it out. I get more points if I send the bird flying directly through the pigs instead of just bursting their bubble things but it's not enough. The grass stuff on the moon isn't enough points to justify using another bird to destroy it. Can anyone help me with this?
I've been retrying this level now for a few times just to check. Using just one bird, it gave me 2 stars once and 3 stars all the other times. That was apparently unrelated to me bursting their bubbles as opposed to hitting them directly, and independent of me shaving some grass. So I guess there's just some fluke - I recommend you just try it a few more times.
As a way of adding variety to your tries, try shooting into them a different direction than the one you've been trying - you can shoot into them from below as well as above. It makes shaving the grass or colliding with the moon easier.