Instructions
Look for the first Star Coin shortly after you go underground; it's in plain sight, above the second tilting ground section, just to the right of the first tilting ceiling section. Wait for the ground to tilt up enough, jump up and grab the coin.
Enter a hidden room through the green pipe just to the right of the game's first tilting girder to find the second Star Coin. Move the girder's right side up by tilting the Wii controller to the left, and then run up and jump to the pipe. Inside the hidden area you'll see some coins, a question block and a goomba trapped in a square of blocks. Don't take the coins yet. In the question block is a P switch; to either side are some invisible blocks, which will help you reach the switch. Jump up and hit the P switch to turn the coins temporarily into blocks. Jump up to the area's higher level (being careful of the goomba, who will be released when you hit the switch), and get the second Star Coin, which is suspended in the air. You need to move quickly before the blocks turn back into coins.
Keep your eye on the bottom of the screen as you approach a group of piranha plants in red pipes, past the halfway flag. You'll see the third Star Coin, just to the right of an seemingly inaccessible green pipe. If you're big, you can get to this pipe by executing a ground pound: jump and press down on the d-pad, and you'll smash through the blocks and reach the green pipe. Go down the pipe, into the hidden area below. Grab the pow block and throw it; the Star Coin, and a bunch of regular coins, will drop down from above, to be collected at your convenience. Now just finish the level.