In this paper it is offered the simplest microscopic model for the description of the nanocomposite material, which is found under action of external electromagnetic (electric) field. At that, the trimer nanocluster embedded in the weakly structured non-dissipative matrix (for example, the polymeric, organic or amorphous types) is modeled as three- center molecular complex with one "excess" tunneling electron and all its centers are considered together with account of theirs ligand environment. The proposed model description is suitable to the trimer nanoclusters of bridge type, and to the nanotrimers such type in which the ions of 3d-metals and theirs oxides have the various oxidation degrees, and also to other three-center molecular complexes of similar types. For posed problem a description of external and internal factors and analysis of their interconnections are performed. Herewith, the given model is also characterized by fact that in addition to the explicit connections of its basic factors they, furthermore, are implicitly interconnected. Then, the mathematical model is formulated so that to contain an optimal number of model parameters for a detailed description of considered factors and its interconnections. In the result of numerical simulation can to identify the electron localization regimes in the trimer nanocluster and the values of controlling model parameters responsible for switching between the obtained regimes.