for (int i = 0; i < indices.Length / 3; i++)
 {
     int index1 = indices[i * 3];
     int index2 = indices[i * 3 + 1];
     int index3 = indices[i * 3 + 2];

     Vector3 side1 = vertices[index1].Position - vertices[index3].Position;
     Vector3 side2 = vertices[index1].Position - vertices[index2].Position;
     Vector3 normal = Vector3.Cross(side1, side2);

     vertices[index1].Normal += normal;
     vertices[index2].Normal += normal;
     vertices[index3].Normal += normal;
 }