For studying 3D stability effects and temperature oscillations induced by thermo-gravitational convection in Czochralski crystal growth at small Prandtl number, a spectral-finite-difference method is employed. In the 2D (axisymmetric) and 3D approximations, the critical Grashof numbers for the onset of oscillations are determined for the parameters corresponding to an international test. The influence of three-dimensionality and the conditions of validity of the axisymmetric approximation are considered.