[en] Part 3 of the paper mostly discusses the problem of justifying the approximation in which the reduced resolvent is replaced by the pole term alone. Imposing additional regularity assumptions on the function v, which specifies the interaction, it is possible to estimate the difference in the corresponding reduced propagators. The result is used to derive the estimate of the deviations from the exponential decay law which results from the pole approximation. With the exception of very small and very large times, the obtained bound is proportional to the fourth power of the coupling constant. The Fermi golden rule was proved for the model under consideration, and the present method compared to the one previously used by Demuth. (author). 7 refs