  We prove a sharp three sphere inequality for solutions to third order
perturbations of a product of two second order elliptic operators with real
coefficients. Then we derive various kinds of quantitative estimates of unique
continuation for the anisotropic plate equation. Among these, we prove a
stability estimate for the Cauchy problem for such an equation and we
illustrate some applications to the size estimates of an unknown inclusion made
of different material that might be present in the plate. The paper is
self-contained and the Carleman estimate, from which the sharp three sphere
inequality is derived, is proved in an elementary and direct way based on
standard integration by parts.
