Instructions
Begin the castle level on World 2.
Play through the first part of the stage as normal. The second stage, where you exit the castle and enter a sandy dune, is where the third and final star coin is located.
Run to the right, avoiding all of the bullet bills that shoot projectiles at you. Eventually, you will come across a platform with a golden P-switch.
Jump on the platform, taking care to avoid the bullets, and jump on the P-switch. Beneath the platform, the sand will crumble into a convex shape and a spiked ball will roll up and down, back and forth.
Jump off the platform and across the newly-formed canyon, avoiding the spiked ball. Just to the right, you will come across a gap with another large spiked ball, along with a rope on the ceiling in front of you.
Jump onto the rope and swing back and forth using the "Left" and "Right" buttons. Press "A" to jump across to the platform to the other side, where two question blocks rest.
Jump beneath the second question block and take the miniature mushroom that appears, shrinking Mario. Run across the platform and run up the next set of stairs. There will be a small gap in the second stair that will lead to the third and final star coin.