Collect 8 red coins. It is essential to finish Bowser in the Fire Sea to collect these 8 red coins. Swim pass the underwater tunnel to the other side, press the ! switch and climb up blocks appeared. Jump from moving post to moving post to collect all 8 red coins.
Swim passes the underwater tunnel to the other side. Notice there are bubbles and rings coming up from it. A star will appear when Mario swim passes 5 rings. It is easy to do so by swimming from water surface down to it. Then use the green ! switch to turn Mario into Metal Mario and collect the star.
There is a manta swimming in the whirlpool area. When Mario swim through 5 consecutive rings coming out from its tail, a star will appear.
Swim pass the underwater tunnel to the other side, hit the blue ! box, turn Mario transparent and swim into a cage underwater where there is a star.
Max number of coins = 106
Mario will see the blue block when collecting 8 red coins for star 3.