Now that you have the three pearls, you need to take them to the triangle islands, which are now marked on your sea chart.
Southern Fairy Island
First, go northeast. You'll probably encounter enemies on the way, so have your boomerang ready. Also, remember to avoid that giant cyclone if you see it. You'll pass Shark Island on the way.
Before going to the southernmost place marked on your sea chart, you can go to the island in the square directly south of it. It's Southern Fairy Island, where you can get an upgrade to carry more bombs. Just bomb the planks and go inside, then walk up to the fairy to get the upgrade.
The Triangle Islands
From the fairy island, just head N to reach Southern Triangle Island. Just walk up to the statue here to place the pearl. Afterward, set the wind N and go two squares north. You'll pass Six-Eye Reef on the way.
After placing the pearl at Northern Triangle Island, you can go east to the fairy island. Use a bomb to get the big rock out of the way, then go inside to get another bomb upgrade. Now you can hold 99.
Then just go southeast. You might have a treasure chart for Eastern Triangle Island. The treasure here is 200 rupees.
Finally, place the pearl at Eastern Triangle Island and watch what happens.