This stage is Beep Block Skyway (4-3). This complete walkthrough will provide you with a 100% video playthrough and a screenshot of where you can find all the green stars. The best power up to use in this stage is Fire Flower Mario as this stage has plenty boomerang enemies. Throughout your play, you should get all the double cherries to activate multiple switches.
Always try to run against the walls so that your copies are as close to each other. This will avoid losing them while playing the stage.
Game Play Walkthrough Video
Green Star 1
You will reach the first Green Star quite easily. The trick is to kill the boomerang enemy first before going for the Green Star.
Green Star 2
After reaching the checkpoint and the stamp. Head right to go through an orange mystery box. Go inside a green pipe next to the three clock timers. Collect all the blue coins after hitting the power switch to unlock the second green star.
Green Star 3
Near the end of the stage, you will need two characters to stand on the switch to reveal the last green star.
After hitting the check point flag, stand on the platform with number 4. You should have 4 characters by this time if you did not lose any to the boomerangs. Rise up and grab the stage stamp.