It was demonstrated theoretically, that organic crystals of TTT2I3 (tetrathiotetracene-iodide) are very prospective thermoelectric materials. The internal structure is strongly quasi-one-dimensional and the thermoelectric transport takes place mainly along the molecular chains of TTT. Numerical results, obtained initially in the frame of the simplified one-dimensional (1D) physical model are very promising. In the 1D model, the weak interaction between the conductive TTT molecular chains is neglected. In order to define more precisely the criteria of applicability of the 1D model, the more complete 3D physical model was elaborated. It is shown that for crystals with rather low degree of purity the scattering on impurities and defects predominates on the scattering on nearest neighbor chains and the 1D model is applicable. For ultrapure crystals, the interchain interaction becomes important and the transport phenomena should be generalized for the 3D case. Also, it is shown that for diminished carrier concentration and rather pure crystals, the thermoelectric figure of merit may achieve rather high values.