  The following problem is studied: describe the triplets $(\Omega,g,\mu)$,
$\mu=\rho\,dx$, where $g= (g^{ij}(x))$ is the (co)metric associated with the
symmetric second order differential operator $L(f) = \frac{1}{\rho}\sum_{ij}
\partial_i (g^{ij} \rho\,\partial_j f)$ defined on a domain $\Omega$ of
$\mathbb R^n$ and such that there exists an orthonormal basis of $\mathcal
L^2(\mu)$ made of polynomials which are eigenvectors of $L$, and the basis is
compatible with the filtration of the space of polynomials with respect to some
weighted degree.
  In a joint paper with D. Bakry and M. Zani this problem was solved in
dimension 2 for the usual degree. In the author's subsequent paper this problem
was solved in dimension 2 for any weighted degree. In the present paper this
problem is solved in dimension 3 for the usual degree under the condition that
$\partial\Omega$ contains a piece of a tangent developable surface. The proof
is based on Pl\"ucker-like formulas in the form given by Ragni Piene. All the
found solutions are generalized for any dimension.
