function tripleProduct(v1, v2, v3) {
    var v = TRIPLE_REGISTER;

    Vec3.cross(v1, v2, v);
    Vec3.cross(v, v3, v);

    return v;
}
The so called triple product. Used to find a vector perpendicular to (v2 - v1) in the direction of v3.
(v1 x v2) x v3.

@method
@private
@param {Vec3} v1 The first Vec3.
@param {Vec3} v2 The second Vec3.
@param {Vec3} v3 The third Vec3.
@return {Vec3} The result of the triple product.