The Three-Eye Reef (A4 on the worldmap) is one of the forty-nine distinct islands in The Legend of Zelda: The Wind Waker. It is one of the many numbered-eyed reefs in the game, and is named so because of the three circular rocks jutting out of the ground that are contained within a rocky barrier. An entry way can be found at the top of the western wall.
After you destroy the enemy ships that have made this area their home, you'll be rewarded with Treasure Chart #32, which can be found on top of the wall. You might need to use your Deku Leaf in order to get to the treasure chest after it appears. The second reward can be found deep within the ocean. You'll need your grappling hook, which you'll send under water to retrieve a Piece of Heart (#35). To find the treasure chest you'll need Treasure Chart #38.