The effective Majorana mass which determines the rate of the neutrinoless double beta ((betabeta)(0v)-) decay, \<m>), is considered in the case of three-neutrino mixing and massive Majorana neutrinos. Assuming a rather precise determination of the parameters characterizing the neutrino oscillation solutions of the solar and atmospheric neutrino problems has been made, we discuss the information a measurement of \<m>\ greater than or similar to (0.005-0.010) eV can provide on the value of the lightest neutrino mass and on the CP-violation in the lepton sector. The implications of combining a measurement of \<m>\ with future measurement of the neutrino mass m(ve) in H-3 beta-decay experiments for the possible determination of leptonic CP-violation are emphasized. (C) 2002 Elsevier Science B.V. All rights reserved.