Relatively recently (A. Amaya-Tapia, S. Y. Larsen, M. Lassaut. Ann. Phys.,306 (2011) 406), we presented a formula for the evaluation of the third Bose fugacity coefficient -leading to the third virial coefficient -in terms of three-body eigenphase shifts, for particles subject to repulsive forces. An analytical calculation for a 1-dim. model, for which the result is known, confirmed the validity of this approach. We now extend the formalism to particles with attractive forces, and therefore must allow for the possibility that the particles have bound states. We thus obtain a true generalization of the famous formula of Uhlenbeck and Beth (G.E. Uhlenbeck, E. Beth. Physica,3 (1936) 729; E. Beth, G.E. Uhlenbeck. ibid, 4 (1937) 915) (and of Gropper (L. Gropper. Phys. Rev. 50 (1936) 963; ibid 51 (1937) 1108)) for the second virial. We illustrate our formalism by a calculation, in an adiabatic approximation, of the third cluster in one dimension, using McGuire's model as in our previous paper, but with attractive forces.