This stage is Switchblack Ruins (Castle-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
When you get to the room with four unlit torches on four corners, light them all up for the first green star.
Green Star 2
Right after the check point flag, keep on going right to discover a hidden passage which leads to the secret green star.
Green Star 3
At the room where there is a green pipe leading the goal pole, hit the invisible block to the back. The final green star sits on top of the wall in the back.
After the second green star, you will hop on a platform which transports you across a big lava pit. At the center, jump to the edge on the right to obtain the stage stamp.