The nature and importance of nonadditive three-body interactions in the ionic OH-(H2O)(2) cluster have been studied by supermolecule Moller-Plesset (MP) perturbation theory and coupled-cluster method, and by symmetry-adapted perturbation theory (SAPT). The convergence of the SAPT expansion was tested by comparison with the results obtained from the supermolecule Moller-Plesset perturbation theory calculations through the fourth order (MP2, MP3, MP4SDQ, MP4), and the coupled-cluster calculations including single, double, and approximate triple excitations [CCSD(T)]. It is shown that the SAPT results reproduce the converged CCSD(T) results within 10%. The SAPT method has been used to analyze the three-body interactions in the clusters OH- (H2O)(n), n = 2,3,3,10, with water molecules located either in the first or the second solvation shell. It is shown that at the Hartree-Fock level the induction nonadditivity is dominant, but it is partly quenched by the Heitler-London and exchange-induction/deformation terms. This implies that the induction energy alone is not a reliable approximation to the Hartree-Fock nonadditive energy. At the correlated level, the most important contributions come from the induction-dispersion and the MP2 exchange energies. The exchange-dispersion and dispersion nonadditivities are much smaller, and for some geometries even negligible. This suggests that it will be difficult to approximate the three-body potential for OH-(H2O)(4) by a simple analytical expression. The three-body energy represents only 4%-7% of the pair CCSD(T) intermolecular energy for the OH-(H2O)(2) cluster, but can reach as much as 18% for OH-(H2O)(4). Particular attention has been paid to the effect of the relaxation of the geometry of the subsystems.