Quantum dynamical calculations for the IR-multiphoton excitation of H_2O,D_2O andT_2O are presented using a generalized Hamiltonian for XY_2 molecules.The energy spectraobtained from this Hamiltonian are in good agreement with those of the experiments.Thelong time average of the transition probabilities and the isotopic effects are discussed in detail, Quantum dynamical calculations for the IR-multiphoton excitation of H_2O, D_2O and T_2O are presented using a generalized Hamiltonian for XY_2 molecules. The energy spectraobtained from this Hamiltonian are in good agreement with those of the experiments. The long time average of the transition probabilities and the isotopic effects are discussed in detail,