A model of charge ordering in crystals comprising trinuclear mixed-valence complexes is proposed, Qualitatively different types of temperature behaviour of the mean value of crystal dipole moment (order parameter) are shown to be possible, These types of dependences lead to one, two and three phase transitions, The phase transitions predicted are elucidated to be of the first and the second kind. The anomalous behaviour of the magnetic moment is shown to arise from non-magnetic electric dipole-dipole interaction between molecules.