Ternary complexes of HNO3 center dot center dot center dot HCl center dot center dot center dot H2O were investigated by ab initio calculations with aug-cc-pVDZ and aug-cc-pVTZ basis sets. The results are analyzed in terms of structures, energetics, and infrared vibrational frequencies. In all minima, neither HNO3 nor HCl becomes ionized. The contribution of the nonadditivity effect, which is significant for hydrogen-bonded clusters, is bigger for the cyclic structures in which HNO3 acts as a proton donor to HCl, although the global minimum contains HNO3 donating its proton to a H2O molecule.