The stability analysis of Finite Difference-Time Difference (FD-TD) schemes can be reduced via the von Neumann approach to the study of a sequence of polynomials with coefficients depending on the physical parameters of the medium and the numerical parameters. Such computations are very tedious in 3D since the simplest medium involves 9th degree polynomials. A Computer Algebra environment has been designed to automize such calculations. This technique is demonstrated on schemes for Maxwell-Debye and Maxwell-Lorentz systems.