Dynamically screened three-Coulomb-wave model (DS3C) is used to study the single ionization of the 3S orbital of sodium by electron impact. Triply differential cross-sections (TDCSs) are calculated in doubly symmetric geometry at excess energies of 6, 10, 15, 20, 30, 40, 50 and 60 eV using a parameterized optimized effective potential. Comparisons are made between recently experimental data and theoretical predictions of the three-Coulomb wave function (3C) approach and distorted-wave Born approximation (DWBA). The angular distribution of the present TDCS is found to qualitatively reproduce the reported experimental data. It is shown that dynamically screened effects are important in this geometry.