It's time to go back to the circus! Take the gondola down to the level where you'll find tilting platforms, Fizzlits, and trapezes. The trapezes can hold four people at a time. They trapezes are already swinging, but you can push left and right to make them swing even more. Keep going to the right past the three Fizzlits and you'll find invisible clouds over one of the trapezes. The first Green Star is up there. Take the gondola to the right down to the midpoint.
Ahead is a large, spinning platform with many Piranha Plants and giant Piranha Plants. Go to the right for a pipe. It leads to a P-Switch. Collect the Silver Coins for the second Green Star. Return and go to the center of the spinning platform. The triple item block has a Mega Mushroom. Kill all of the Piranha Plants for lots of coins. Go to the back and bounce on the orange bouncy platform to reach the large stairs. The stamp is to the left. If you can climb the wall, you'll find Captain Toad with a random item block. Otherwise, go through the clear pipe. You'll land on tilting platforms with spike bars. Swing on the trapeze and hit the Green Ring to make Green Coins appear. It doesn't matter which spot you swing from on the trapeze, but you must swing quickly and jump when the opportunity presents itself to collect all eight. This will get you the third Green Star. Take the gondola down to the trapeze to the flag. Time your jump carefully as there is nothing under the trapeze. It can be a little difficult to reach the top of the flagpole from here, so take your time swinging.
The stamp is of Luigi.