To complete the third World, you have must complete one goal: You must rescue three children and bring them to the stone gate deep underwater in order to gain access to the underwater temple, defeat the boss and rescue the fourth and final child. Easy to say, difficult to do.
 Using items
In order to accomplish this goal, you're going to need to make use of four tools; The Passing Hoop, the Key, the Talisman, and the Holding Bag. Doraemon is limited to carrying only one item (or person) at a time. However, when Doraemon is holding on to the Holding Bag, up to two items (or people) will follow Doraemon around as he swims from room to room, making it an essential item.
You can find one of the four items wherever a green "!" is located on the map. The items are positioned in each of those rooms randomly. The item in the upper right corner can be difficult to reach unless Doraemon swims down to the room below, and then swims straight up. The more Doraemon swims up, the higher he will jump out of the water. Remember to press to grab the item.
 Reaching the treasure chests
There are a total of seven treasure chests. Two are behind walls, two are guarded by an octopus, and one is both behind a wall and guarded by an octopus. Only three of these chests contain the kidnapped children, while the others contain deadly sea serpents. The only thing that is capable of defeating a sea serpent is the talisman. Doraemon must carry the Talisman and touch the serpent to destroy it. No amount of shooting will eliminate it.
The octopus, on the other hand, is surprisingly easy to defeat. You'll just have to be prepared to take a little bit of damage while fighting it. As soon as you arrive in an octopus chamber, two of his tentacles will reach out to Doraemon. They will latch on and start draining his health. You have no choice but to endure it while you rapidly fire shots at into the tentacles. Pump eight shots into the tentacles as fast as you can, and the octopus will be defeated. Don't relax right away, the room will begin to fill with regular enemies.
Once an Octopus is removed, it's easy enough to access the treasure chest with the key. If the chest is behind the wall, you'll have to do a little more work. In order to pass through the flat walls the block a few of the passages, you'll need to use the Passing Hoop. Simply touch the wall with the Hoop and the wall will open. When you pass through to the other side, the wall will remain open. However, once you leave the room and return, the wall will be back. Therefore, it's important that you bring the Hoop along with you so that you do not become trapped behind.
 Bonus room, Candy, and Ghosts
As you fight enemies underwater, they will leave many items behind. The most valuable of which is the diamond, which will destroy all of the existing enemies on the screen. You may also find gold bars. If you collect enough diamonds and gold bars, you will be transported to the bonus room. In the bonus room, dorayaki and skulls will come pouring out of the stone structure. You will enter the room with full health. Every time you are touched by a skull, you will lose health. The bonus stage will end when you collect 20 dorayakis (worth 500 points each,) or if you lose all your health by touching skulls. No matter what happens, you will return to the game with full health.
The other thing to be on the lookout for is the two Genki Candies in the this World. One is available in plain sight, but very deep underwater. The other one is hidden behind a wall, so you'll need the Passing Hoop to collect it. Finally, one thing to watch out for is the thieving ghost. When you stay in one room long enough, a ghost will appear. This ghost can't hurt you, but it like to steal things. It will try to take any of the items or people that are in the room with you. If it grabs an item, it will move it to another part of the sea, usually one screen away but not necessarily in the direction it left. If you manage to leave the room before the ghost leaves, the item will remain in that room (or follow you if you have the Holding Bag). If the ghost can't find an item to steal, it will sometimes steal an enemy. The ghost can be destroyed by touching the diamond.
 General strategy
In order to make the best use of your time, you are going to want to find the Holding Bag, and carry the Passing Hoop and the Key along as much as you can. This means you'll have to leave the Talisman behind, so leave it somewhere that is a) handy and b) out of the way. If you have to pass through the room with the Talisman, the game will force one other item to stay behind in the previous room.
Begin by swimming around with the bag, key, and hoop. Access the treasure chests in any order that you like. If you find a child, remember where it is, and leave him behind. If it's a serpent, leave the room immediately. The serpent may appear in rooms other than the one you found it in, but don't worry. Simply continue to swim back to the room where you left the Talisman. Use the Talisman to defeat the serpent, then continue seeking out treasure chests.
Once you have discovered all of the children, use the Holding Bag to transport too of them to the water gate at the bottom of the map (below the middle octopus). Then return for the last one, and use the bag to transport the remaining child and the Talisman to the water gate. When all three children are assembled in the water gate room, the gate will open. At that time, you will gain access to the underwater temple. The only item that you will want to bring inside is the Talisman.
 The underwater temple
The underwater temple occupies the lower right corner of the world. You will have to travel through all nine chambers to reach the final room where Shizuka is being held captive. A unique enemy appears in its halls, but it is no more difficult to deal with than any you have encountered previously. After passing through four rooms, you will pass through three chambers, each of which contains a sea serpent. If you brought the Talisman along, you have little to worry about and can simply deal with them as usual. If not, do your best to swim quickly through the room before the serpent moves around too much.
When you reach the final chamber, you'll see Shizuka trapped in a cage in the middle of the room. A water spirit known as Poseidon will appear to attack you. He will fire boulders out of the top of his head. The best way to deal with him is to attack him in one corner. When the spirit guess too close, swim to the opposite corner and resume your attack. Repeat this strategy until you hit him with 16 shots. Once you do, the spirit will be defeated, and the last child will be defeated. Doraemon will have successfully rescued all four children, and you will be treated to the game's ending.