Green star 1- At the beginning of the level there are wooden boxes along the side of the wall. Wreck these with a power to reveal the green star.
Green star 2- After you have completed the first board, there is a group of enemies swimming in circles. Behind them, they have coins, a fire flower, and the second green star. Just wait for the enemy to swim next to you and take the star.
Green star 3- Towards the end of the level, there are two Rammerheads spinning around a hole in the wall. In the hole it a pipe containing the third star. Going through the pipe would risk hitting the Rammerheads on the other side, so you can alternatively head to the top of the pipe and throw fireballs through it.
Just before the final pipe, there are Rammerheads guarding a small opening. Avoid them and enter the opening to retrive the stamp.