This stage is King Ka-thunk’s Castle (5-Castle). This complete walkthrough will provide you with a 100% video playthrough and a screenshot of where you can find all the green stars. You will need the Cat Mario to collect the stamp and green star for the stage.
Game Play Walkthrough Video
Green Star 1
After trigger the hollow green star after the football enemies. Jump on the platforms to collect all the coins and claim your green star.
Green Star 2
You will see a Green Star inside one of the square block as you progress through the stage. The green star rotates between two squares so just wait if you miss it.
Green Star 3
You will see an orange mystery box at the end of the stage. Do not go in yet and instead climb the wall to the left and get your final green star.
Climb to the wall to the right before the second green star. The stamp is sitting at the top of the wall.