This stage is Conkdor Canyon (2-1). 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 treasure room which can net you a handful of coins worth about 1UP.
Game Play Walkthrough Video
Green Star 1
The first green star is located near the beginning of the stage, simply jump onto the platform and climb the wall as Cat Mario to obtain the first green star.
Green Star 2
When you reach to Conkdor bird surrounded by stone blocks, stand next to the right most block. The green star is hidden inside and you can obtain it when the Conkdor bird breaks it.
Green Star 3
At the end of the stage after you obtain the stamp, turn right instead of heading into the finish line. Kill the Conkdor bird attacking Toad. The grateful Toad will then reward you with the last Green Star.
You can obtain the Stamp at the end of the stage, you won’t miss it.
Other Notable Secrets
Secret Treasure Room
The secrete treasure room can grant you enough coins to warrant an 1UP. Climb up the side of the mountain after you wrap around a curve. Enter the room through the golden pipe. This screenshot shows where to climb.