World 3 contains a single Stamp to collect in every level.
Towards the end of the level, there is a large pit in the ice. Walk around the edge and grab the Stamp!
Enter the clear pipe and jump on the donut lift. Allow it drop, but jump off at the right moment to grab the Stamp. Alternatively, climb up the grate in your Cat form.
Hop onto the love seat near the start of the level to have it float up to the Stamp above.
Peach required! To the left of the area with the spinning platforms is a Peach switch. Hit it to reveal this Stamp!
Before you reach the small sunken ship, swim up to find a hidden room. Here you’ll find a Stamp and the Cannon Block.
Use the jump panel to launch yourself up to the bouncy mushrooms. These will take you to the Stamp at the top.
After the third Green Star, hop onto the next lift and wait until it goes all the way to the left. Before it moves again, jump to the next lift. Let this one take you to the Stamp.
Right before you enter the green pipe that takes you to Pom Pom, drop down to the ledge below to find a Fire Bro. guarding the Stamp.