Instructions
Use the three revolving blocks to reach the red platform directly above them. Run and slide under the blocks there and hit them. One of them contains a bean stalk, which will rise upward into the sky. Climb it to get to the secret area up there. Jump onto one of the groups of question blocks orbiting the multicolored blocks, and between them you should see the first star coin. Jumping onto the leading question block will make this a lot easier, since you'll automatically stay on top of it when the blocks turn the corner. Drop down and grab the coin. If you have the propeller suit, you can just jump and shake the controller to fly up and get the coin. You will have to go over the top since the bottom part of the area between the multicolored blocks is lined with invisible blocks.
Look for the second star coin on the far right of the second large spinning section of ground after you come down from the secret area. This is a difficult jump; waiting too long will send you falling off the level. It's much easier to do with a propeller suit. You can also jump on the koopa here and throw its shell at the coin. If you hit the coin, you get it without having to actually touch it yourself.
Look for the last coin above the small rotating red sections just before the end of the level. If you have a propeller suit, getting this coin is as easy as jumping up and flying to it. If not, you'll have to make use of the para-koopa flying up and down below the coin. Wait until he's high up in his course, then run and jump on him. Hit the "2" button exactly as you land on him, and you'll get a boost to your jump. That jump should be just enough now to reach the final coin.