A lattice model is proposed in order to investigate the superfluid phase transition and the phase separation in liquid mixtures of He3 and He4. The Matsubara-Matsuda model for pure liquid He4 is generalized. An effective Hamiltonian is expressed in terms of spin operators with the magnitude unity. It is solved with the aid of molecular field approximation. A phase diagram thus constructed reproduces characteristic features of liquid He3-He4 mixtures. Namely, the lambda line, the critical point of the phase separation and the stratification curves are obtained in good agreement with experiments. In addition, it is shown in a unified manner that the lambda temperatures exhibit a qualitatively correct dependence on He3-concentration and pressure.