This stage is Puffprod Peaks (2-2). This complete walkthrough will provide you with a 100% video playthrough and a screenshot of where you can find all the green stars.
Game Play Walkthrough Video
Green Star 1
The first green star is hidden on the left side of a wall of moveable platforms. Check the left side instead of the right to grab the star. It is better to use Cat Mario to climb up the wall.
Green Star 2
The second green star can be obtained from a smiley face cloud near moveable platforms. Blow into the GamePad microphone to ride on the platform. Grab the Super Star and pass the stage easily and get the second green star.
Green Star 3
The last star is hidden under the edge where there is a Maneating plant. Go under the platform and get on the moving platform to get the star. You will have to keep on blowing into the microphone so that it does not disappear on you while waiting.
The stamp is hidden under the colored platforms early on. Slide into the right platform to get into a secret room with the stamp of the stage.