[en] General description of the three-particle spectrum with its two main properties - the Thomas theorem and the Efimov effect is given. The spectrum of levels generated by only two-particle (particle + pair bound state) contributions to the unitarity condition is shown to have none of these properties. With the account of the three-particle intermediate states in the unitarity condition the N/D equations reproduce both the Thomas theorem and the Efimov effect. The quantitative calculation of a three-particle spectrum by means of N/D equations, making use of only on-shell characteristics of the two-particle system, is in reasonable agreement with the solution of the Faddeev equation