World 1-2

Coin 1 (before checkpoint)

You'll spot the first coin just after entering the underground area. You can easily reach it from the platform below when the left side tilts upward.

Coin 2 (before checkpoint)

When you reach the first controlled tilt-platform, you'll spot a pipe in the top-right. Reach it by tilting the right side of the platform up and head inside. There, an item block reveals a "P" switch, which turns the coins into platforms and visa-versa. To reach it, hit a couple of invisible of invisible blocks to its left, and try not to grab the coins while leaping onto them. As soon as you hit the "P-Switch", use the newly formed bricks to reach the top of the room, where you'll find your coin.

Coin 3 (after checkpoint)

After the checkpoint, you'll find a bricked-in green pipe (as well as the coin just beyond, which you can't reach). Butt-stomp through the bricks to and enter the pipe. Underground, hit the P-Switch on the right side of the room to cause the coin to drop down.