It's extremely hard to get through the fortress with these birds.
3 stars is extremely hard to get.
Send the ice bird into the hollow wood block directly below the first flag. With enough force he will freeze the wood beam protecting the massive pig. Now the key to this strategy is to fire the blue bird at the inside edge of the atmosphere towards the pile of popcorn and kernels. Split the bird just before he enters the atmosphere so one blue bird hits the popcorn kernels. With a little luck this should send debris flying towards top planet and take everything out.