The geometries and vibrational properties of the low-lying electronic states of neutral and anionic of M-3 (M = P, As, Sb, and Bi) are studied using the coupled-cluster singles, doubles, and noniterative triples (CCSD(T)) method as well as the density functional theory (B3LYP-DFT) method. For P-3(-), the (1)Sigma(g)(+) and X(3)A(1)'(D-3h) states are almost degenerate. The X(3)A(1)'(D-3h) state, however, turns out to be the lowest state for As-3(-), Sb-3(-), and Bi-3(-), and the adiabatic excitation energies of the (1)Sigma(g)(+) state are 0.6, 0.9, and 1.0 eV, respectively. In the anionic trimers of all four elements, another singlet state, (1)A(1)(C-2nu), is located about 0.3-0.4 eV above X(3)A(1)'(D-3h); the energy gap between these states is compared to the splittings between the first two peaks in the photoelectron spectra of these anions. For all of the neutral trimers, the adiabatic and vertical energetic splittings between the Jahn-Teller components of the (XE)-E-2" and E-4' states are calculated to be only 0.04-0.08 eV. Another quartet state, (4)A(2)" is 0.4 eV higher, almost equal, 0.2 eV lower, and 0.3 eV lower in energy than the E-4' state in P-3, As-3, Sb-3, and Bi-3, respectively. All of the features of the main peaks in the photoelectron spectra of the anions observed to date are explained by using calculated geometries, vibrational frequencies, and excitation energies. In addition, a number of peaks are predicted that have not yet been observed experimentally.