intersection static method
Find the intersection point between the three planes a, b and c and
copy it into result.
Implementation
static void intersection(Plane a, Plane b, Plane c, Vector3 result) {
  final cross = Vector3.zero();
  b.normal.crossInto(c.normal, cross);
  final f = -a.normal.dot(cross);
  final v1 = cross.scaled(a.constant);
  c.normal.crossInto(a.normal, cross);
  final v2 = cross.scaled(b.constant);
  a.normal.crossInto(b.normal, cross);
  final v3 = cross.scaled(c.constant);
  result
    ..x = (v1.x + v2.x + v3.x) / f
    ..y = (v1.y + v2.y + v3.y) / f
    ..z = (v1.z + v2.z + v3.z) / f;
}