World 3-5: Pipeline Lagoon
Before heading through the clear pipe by the red arrow, swim up to find a star inside a small grotto.
Stamp: Cheep Cheep
After the long clear pipeline, you'll be in a second watery area. Swim up from the red arrow to find a hidden area. Use the rising tide or wall jumps to reach the ledge where a stamp sits.
Grab the Bomb Box from the opposite ledge and descend to the sunken ship. See the cracked wall to the left? Use the Bomb Box to break through and reveal a warp pipe. In here is a secret area with tons of coins, bonus time, and a star.
Just after the sunken ship, you'll reach three stacks of Spiny Cheep Cheeps. Carefully swim into the tunnel on the far right, then swim straight up to find a secret area. Take a right and ride the tide to another star.
World 3-6: Mount Must Dash
When the path splits, take the lower one. Across the falling platforms is the level's first star.
Right after the steep hill past the checkpoint, look for this group of flying enemies - they're patrolling a mystery box. Hop inside, and you'll have 10 seconds to catch a rabbit. Do it to earn a star.
After leaping the gaps with rows of coins, make sure you hit this jump pad. Doing so will bounce you up to a stamp.
Just beyond is the level's biggest jump. Hit it hard and with a lot of speed to get lots of airtime and reach this high altitude star. You may want a Tanooki Suit to make the jump a little easier.
World 3-7: Switchboard Falls
As you head down the first big slope on a switchboard, jump to the walkway above. Use the warp pipe to reach a secret area behind the falls. As you descend in this area, stick to the path of coins to grab a star on the way down.
When you drop to the switchboard right of a waterfall, press left and duck. You'll skirt under the ledge and reach the star beyond.
During the final set of switchboards, you'll drop to a lower railing. Keep pressing right, and jump over the enemy that rolls toward you. At the top of this slope is another star.
Now ride the switchboard to the left, and jump to the lowest switchboard you see. Ride this to the end of its track to find a stamp.
World 3-Special: Captain Toad Makes a Splash
Hop into the water to grab this from the corner.
Head through the little space in the water, then enter the clear pipe. Bear right to reach the upper right ledge where a star sits.
Return to the clear pipe and take the other path. Follow the ledges up here as they wind around, then drop onto the green walkway. Follow it to grab another star.
Hop into the clear pipe, then drop into the hole on the other side. The switch here raises the water, bringing wooden planks with it. You can now reach the star just outside of your current position.
Now simply follow the raised planks and head up the slope to the final star.
World 3-Train: The Bullet Bill Express
As soon as you arrive, catch up to the train and jump to the rear ledge to grab this.
Shortly onto the train, a stack of Goombas with a star on top will approach. Take them out to grab the star.
Use the warp pipe to reach the other train. Break the box near the engine to free a Toad. He'll give you a star as thanks.
Stamp: Pom Pom
Back on the first train, approach the engine and drop to the lower ledge. Take out the Fire Bro. to reach the stamp he was guarding.
World 3-B: A Banquet with Hisstocrat
Defeat Hisstocrat to earn this.