World 3-1
Slide down the pipe to start the level. The first Star Coin is past the checkpoint. When you see a red pipe next to two green pipes, head down the middle pipe. Swim to the right, grab the Star Coin, and go out the pipe on the right. For the second Star Coin, wait until the big fish starts heading towards the right, and grab it. To make the third Star Coin easier to grab, hit the switch. If you don't, you can always destroy the blocks, but the fish might get you when you grab the coin. Swim as fast as you can and head up the pipe to finish the level.
World 3-2
Head right, and when you hit the checkpoint, go down the yellow warp pipe. Grab the first Star Coin and head out the green warp pipe. Head across the balancing platforms and jump to the second Star Coin. If you head into the green pipe when you get on the moving mushroom, you can get a lot of coins and it leads to the alternate exit, which leads to World 3-B. Otherwise, keep heading right. Grab the third Star Coin when the mushroom is extremely tilted and finish the level.
World 3-B
Head right until you come across a red warp pipe that is by itself, below an overhanging red pipe. Go left and up the red pipe on the left side. Grab the first Star Coin, go back down, right, and up. For the second Star Coin, head right slightly and wall-jump between the red pipe and the green pipe to go right and grab the coin. Let yourself down and continue right. Grab the third Star Coin and finish the level.