Triangle Island

In The Wind Waker, the Triangle Isles are three islands shaped like triangles, and also in such a formation in the sea that if one were to draw lines connecting them, the result would also be a triangle. They serve the purpose of holding three statues of the goddesses, and one of three different pearls that correspond to the goddesses can be placed in their arms. This is the key to raising the Tower of the Gods that Link must enter during his quest across the Great Sea.

Eastern Triangle Island
Eastern Triangle Island has the statue of Farore, which requires that Farore's Pearl must be placed on it for activation.

Southern Triangle Island
Southern Triangle Island has the statue of Nayru, and as such, Nayru's Pearl must be placed within its arms.

Northern Triangle Island
Northern Triangle Island has the statue of Din, upon which Din's Pearl must be placed. Link can also use the Treasure Chart he received from the Six-Eye Reef to obtain the Big Octo Chart here.

A Link to the Past Connection?
It has been noted by some members of the community that the positions of the Triangle Isles are similar to the positions of the three temples that housed the Pendants of Virtue in A Link to the Past (e.g. the Eastern Triangle Isle is similarly placed to the Eastern Palace in A Link to the Past, which held the Pendant of Courage, an attribute Farore is commonly associated with).