Power of Three - Diana Wynne Jones

Ayna could predict the future.
Cari could find what was lost.
Gair thought he was ordinary.

The three children of Gest, the chief of Garholt, know the perils of the Moor on which they live. The Dorig, their people's enemies, are cold-blooded, fierce underwater creatures who terrify anyone unlucky enough to happen upon them. The Giants are dangerous and violent.
But it's not until their home is invaded that Gair learns of a dying curse that endangers all three peoples of the Moor. A curse that ordinary Gair, with the help of his extraordinary brother and sister, may be able to break, but only at the most dreadful risk to all three, and to the Moor itself.