This stage is Tricky Trapeze Theater (5-2). This complete walkthrough will provide you with a 100% video playthrough and a screenshot of where you can find all the green stars. We will also note a secret 1 UP.
Game Play Walkthrough Video
Green Star 1
You will find the first green star on the right, you will have to use the trapeze to jump on top.
Green Star 2
When you hit the big circle of Piranha plants, head to the right and enter a green pipe. You will play a mini game of blue coin collection to reveal the green star.
Green Star 3
When you see the green star trigger, trigger it to play a game of green coin collection. You will obtain the third green star after collecting all the coins.
After you use the jump pad to get away from the Piranha plants circle, move to the left of the platform to collect the stamp.