The optimized geometries and binding energies (BEs) for the linear and triangular isomers of the beryllium trimer have been recently studied through benchmark multireference averaged quadratic coupled clusters calculations using very large complete active space self-consistent field (SCF). The largest augmented correlation-consistent atomic basis set for Be was used (aug-cc-pV5Z). The BEs are 26.1 and 16.1 kcal/mol for the equilateral and linear isomers, and these were corrected to account for the basis set superposition errors (Amaro-Estrada et al., J. Chem. Phys. 2011, 135, 104311). In the light of the very small (2.4 kcal/mol) BE of Be2, these large interaction energies for the trimers must be due to the nonadditive collective effects, which we set out to determine here using state-of-the-art quantum chemical methods. The nonadditive three-body corrections proved to be extremely large. In fact, their relative weight when divided by total BEs is about 94% for both isomers. We found that in this case, the two-body and the three-body terms go in the same direction, both contributing to the stabilization of the trimers, which yields the rather large BEs for both isomers. Considering that Be3 contains one trimer and three pairs, this would mean that for solid beryllium (where the total number of dimers and trimers become virtually the same), we may confirm the original assumption of Kolos et al. (Chem. Phys. Lett. 1976, 41, 431), to wit, that nonadditive effects for solid beryllium are conspicuously large. © 2012 Wiley Periodicals, Inc.