Using statistical mechanical perturbation theory, the second-order average current density response is calculated for magnetic field-free classical plasmas. A dynamical fluctuation-dissipation theorem is then derived, thus establishing a connection between triplet microscopic current-current correlations and quadratic response functions; it also leads to a static fluctuation-dissipation theorem which provides a dielectric description of the equilibrium ternary correlation. A comparison of the latter with its expansion in terms of the Mayer pair correlation clusters is discussed.