A non-linear model for time-dependent occupation of lattice gas sites is studied. A self-consistent approximation for the diffusion operator is proposed and studied in discrete and continual forms. It is shown that self-organization effects in the spinodal region lead to a spontaneous transformation of an unstable uniform ground state into an array of self-assembled islands. The relationship between the model and the theory of first-order phase transitions is discussed. The model is applied to the study of self-organization in three-dimensional adsorbates with attractive lateral interactions. A special emphasis is given to the description of a spontaneous islanding during molecular beam epitaxy and related growth techniques. It is shown that the kinetic parameters strongly influence the morphology of space-ordered configurations of the system.