Photons have a momentum given by \hbar k (where \hbar is the reduced Planck constant and k the photon wavenumber), which is conserved in all atom-photon interactions.
For cooling to occur along all directions, the atom must see this friction force along all three Cartesian axes; this is most easily achieved by illuminating the atom with 3 orthogonal laser beams, which are then reflected back along the same direction.