Picosecond pulses of Nd:YAG laser radiation are used to study third-harmonic generation in nanoporous boron silicate glass. Effective-medium approximation with Bruggeman-type effective dielectric constant is shown to provide an adequate description of the main tendencies of this process and to offer strategies to control third-harmonic generation. Lamellar refractive-index inhomogeneities in porous glass with a spatial scale much larger than the size of the pores are revealed by polarization dependence of third-harmonic generation.