World 2-2

Coin 1 (before checkpoint)

Immediately before the checkpoint is a shaft of quicksand--leap into it and ride it down to a hidden, underground area. Once there, hit the P-Switch to drop down the shaft to a warp pipe. Once through, ignore the first green pipe (that's the exit) and continue onward to a yellow, upside down switch. Hit it to make some platforms appear, which will allow you to reach the coin.

Coin 2 (after checkpoint)

Hit the left-most block here to reveal a beanstalk (to both hit the block and reach the beanstalk, leap from the nearby moving platform at its highest point). After climbing to the clouds, ride the moving platform and grab (but don't use!) the P-Switch. Hold onto it until the platform moves beneath the large coin, at which point you should use it, causing the coin to drop right onto you.

Coin 3 (after checkpoint)

Near the end of the level, you'll find a small warp-pipe that only tiny-Mario can reach after tossing the barrels away (you can find a tiny-mushroom via the warp pipe just before, to the left of a pair of moving platforms). Hit the P-Switch after warping to reveal a path of blue coins showing where to wall-jump in order to reach the coin here.