Dispersion relation for $3\to 3$ forward amplitude and generalized optical theorem

Investigation of analytical structure of the three-particle forward scattering amplitude with respect to energy variable of one of particles is performed. The results obtained make it possible to draw the conclusions on crossing properties of the amplitude and to derive the generalized optical theorem relating the discontinuity of the amplitude to the distribution function of inclusive process. For a special case when two oil three particles are of zero mass dispersion relation is proved.