To reach the first Star Coin, stomp the Lakitu at Point 1 and hop onto its cloud.
Follow the coins to the right of the Gold Ring to find the Star Coin.
When you approach the Red Ring at Point 2, a Lakitu appears in the background. Either pass through the Red Ring and use the Blocks and toadstools to reach the red coins, or wait for the enemy to arrive and commandeer its cloud.
The second Star Coin is located in a hidden area. To find it, stomp the Lakitu near Point 3 and use its cloud to fly down to the yellow pipe below the walkway.
After you pass through the pipe, stomp the ? Switch on the ground to reveal four Roulette Coin Blocks. Use the Roulette Coin Blocks to reach the Star Coin before the ? Switch wears off.
The third Star Coin is floating below the platform at Point 4. Stomp the nearby Lakitu, then use its cloud to fly down and collect the Star Coin.