This stage is Mount Must Dash (3-6). This complete walkthrough will provide you with a 100% video playthrough and a screenshot of where you can find all the green stars. Remember to grab a secret bell on the tree at the beginning of the stage.
Game Play Walkthrough Video
Green Star 1
At the first fork of the stage, go for the lower path to grab the first green star.
Green Star 2
After the check point, move right and jump down the gap for the purple mystery box. Be careful not to jump too fast as you can possibly miss the star.
Green Star 3
The third green star is available after the stamp. There are two ways to get this star, the first way is via Peach + the jump hover ability at the spring pad. The second way is easier to use a Cat Mario climb up the mountain for the last green star.
After the second star, you will come to a spring pad where you have to bounce off two more for the stamp.
Other Notable Secrets
At the beginning of the stage, remember to climb up the tree to the left and grab the bell. The bell is very useful for getting the third green star.