This stage is Switchboard Falls (3-7). 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
While riding the ramp, you will see a platform with a green pipe and time clocks. Go inside the pipe. You will then fall with a water fall. The first green star sits at the end of the fall.
Green Star 2
After the check point flag and where you have to pass through a shadow wall. You will fall onto another switch. There is a hidden area to the right where you can access by crouching. You will then find the second green star.
Green Star 3
When you get to the final set of switches, ride the middle one all the way to the right for the final green star.
Shortly after the third green star, you will quickly hop to another switch at the bottom to access the stamp.
Other Notable Secrets
After riding the first big switch, head left and climb. At the center of the fifth platform is a 1-Up in an invisible item block.