We explore the interplay between various interaction and hopping terms, in particular the effect of O-O hopping, tppo on the charge instabilities and phase separation in the three-band extended Hubbard models. Within a weak-coupling mean-field approach, we find that in two dimensions (2D), tppo moves the phase separation boundary appreciably to higher dopings. This would increase the window of doping over which charge-transfer-mediated superconductivity can occur. Considering a larger phase space, we also obtain a more complete picture of the coexistence lines and spinodal decompositions, and determine the charge-transfer-instability critical point. We also consider the 1D case: tppo is found to play a similar role, but the phase separation occurs at significantly lower dopings compared to 2D. This may be of interest to recent Monte Carlo calculations.