  The three-band model with the O-O direct hopping near to unit filling is
considered. We present the general procedure of reduction of this model to the
low-energy limit. At unit filling the three-band model in the charge-transfer
limit is reduced to the Heisenberg model and we calculate the superexchange
constant. For the case of the small electron doping the three-band model is
reduced to the $t-J$ model and we calculate electron hopping parameters at the
nearest and next neighbors. We derive the structure of corrections to the $t-J$
model and calculate their magnitude. The values of the hopping parameters for
electron- and hole-doping differ approximately at 40 %.
