PACS: 71.70.EjThe influence of the external magnetic field on the phase states of a crystal consisting of trimeric mixed-valence clusters is examined. The initial model takes into account the intracluster Heisenberg exchange, electron transfer, as well as the Zeeman and intercluster dipole-dipole interactions. The conditions are elucidated under which the external magnetic field facilitates or suppresses the charge-ordered state. The behaviour of the system in the magnetic field is shown to give information about the key parameters of charge-ordered crystals.