This stage is Snowball Park (3-1). 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
The first star is located inside the hole in the wall behind the flying enemies. Jump on the heads to get into the hole.
Green Star 2
About half way of the stage through a big sheet of ice. Jump towards the left side to find a rabbit holding the green star.
Green Star 3
The final green star is near the end of the level. The green star is in a little hole in the cave behind the trees. You will need either the Propeller box or the Tanooki Suit Mario to get it.
After collecting Green Star #2, head to the right and you’ll see the Stamp on a thin strip of ice in a circular shape.