Equilibrium structures and several other ground state properties of linear and cyclic (HCN)3 clusters were investigated with the aid of ab initio methods applying extended basis sets and including electron correlation. Computed equilibrium structures, stabilization energies, dipole moment, vibrational spectra and infrared intensities are compared with available experimental data. Corresponding results are also presented for the HCN monomer and the linear (HCN)2 dimer. Modifications of monomer properties induced by cluster formation are monitored and discussed. In agreement with our previous self-consistent field studies and with a recent vapor phase spectroscopic experiment, both the linear and the cyclic trimer are true minima on the energy surface and are almost equally stable.