We develop a three-dimensional numerical model for an ensemble of molecular clouds moving in the fixed gravitational potential of a galaxy. This scheme is a modification of the widely known model of Oort and includes different processes of coagulation and fragmentation of clouds under pairwise collisions, interaction of clouds with the diffuse interstellar medium, and also feedback: the breaking up of clouds into small fragments under the action of stars arising in them. This model makes it possible to study the influence of various parameters of both the galaxy itself and the ensemble of molecular clouds on the process of large-scale star formation connected with giant molecular clouds and on the temporal changes of the global structure of the interstellar medium. We give as an example a computation of the evolution of the energy characteristics of an ensemble of molecular clouds in a spiral galaxy.