Star Coin 1: Not long after the midway point, there is a P Switch on the ceiling. Mario and co. should activate the P Switch to both summon Coins and stop the nearby Warp Pipes from blowing wind, then follow the track of Coins to an upside-down Yellow Pipe. This Pipe leads to an area where the first Star Coin is floating high in the air. Mario and co. should hit one of the ? Blocks (both if not Super) to get a Penguin Suit and freeze the nearby Cheep Cheep. This gives Mario and co. a platform to jump from and collect the Star Coin.
Star Coin 2: Not far from the Pipe that leads from the area with the first Star Coin, the second Star Coin is floating in between two Green Pipes. Mario and co. must swim down to it while avoiding any Bloopers or Cheep Cheeps.
Star Coin 3: Just before the exit Pipe at the end of the level is a platform protruding from the ground. Inside this platform is a hidden area, which contains the third Star Coin.