We report the results of a theoretical study of the nitride trimers (i.e., M3N3 With M = Al, Ga, and In) focusing on their structure, stability, and vibrational properties. The calculated results reveal a distinct structural difference between the most stable isomers of Al and those of Ga and In, which can be explained in terms of the evolution of the relative strengths of metal-nit+ogen and metal-metal bonds in going from Al to Ga to In. It is also shown that the strength of the nitrogen-nitrogen bond still dominates the preferred fragmentation path for these clusters and that fragmentation occurs at a lower energy than ionization. Assignments to the frequencies of the various normal modes are made in terms of those of the MN monomers (Al3N3) or in terms of those of weakly bonded (M-3)(+) and (N-3)(-) subunits (Ga3N3 and In3N3).