The first Star Coin is located near the start of the level, but the walkway prevents you from reaching it. Follow the path until you reach the Blocks at Point 1, then ground pound through the Block on the left.
The remaining Block should slow the Goombas bouncing in from the right. Bounce along the pink toadstools to the left, and use your fireballs to clear out the enemies on your way to the Star Coin.
When you pass through the Red Ring at Point 2, eight red coins appear on the surrounding platforms. As you collect the coins, take care to avoid the Goombas as they bounce off of the nearby toadstools.
Enter the green pipe to reach a secret area high above the platforms.
When you land, hit the ? Block to the left. When you do, it transforms into a Flying ? Block. Wait for it to move near the remaining Blocks, then hit it to lock it into place. The used Block can now serve as a platform, allowing you to reach the remaining Blocks.
The second Star Coin is located high above the pink toadstool at Point 4. Wait for a Goomba to step onto the toadstool, then stomp it as it’s launched into the air. Use the extra height to bounce up to the Star Coin.
Only Mini Mario can reach the level’s third Star Coin. Ground pound the ? Block at Point 5, then dash to the right as the Mini Mushroom drops to the platform below the walkway.
Hop over the pipes at the end of the walkway to catch the Mini Mushroom as it emerges from the tiny passage. As Mini Mario, head back to the left and collect the Star Coin under the walkway. When you do, take care to avoid the Goombas emerging from the nearby pipe and any enemies above you.