Four isotopologues of the gas-phase complex HNO3-(H 2O)3 have been observed by microwave spectroscopy in a supersonic jet. Rotational and nuclear electric quadrupole coupling constants have been obtained and the experimentally derived inertial defect has been used to infer a near-planar geometry for the complex. The data identify the observed species from among several structures predicted by theory, favoring a 10-membered ring geometry with the HNO3 hydrogen-bonded to the first water, a series of water-water hydrogen bonds, and ring completion with the third water acting as a hydrogen-bond donor to an unprotonated HNO3 oxygen. This structure corresponds to the lowest energy form predicted computationally in several prior studies as well as in this work using the MP2/6-311 ++G(2df, 2pd) level/basis set. Although its observation does not rigorously establish its status as the lowest energy form, the concurrence between the predicted low-energy conformer and that observed in the ultracold supersonic jet strongly suggests that it is indeed the minimum-energy structure. The a-type spectra show evidence of internal dynamics, likely resulting from large amplitude motion of one or more of the water subunits. This complex represents the third step in the sequential hydration of HNO3, and both the theoretical structure and experimental 14N quadrupole coupling constants have been used to track the degree of ionization of the acid as function of hydration number. Based on 14N quadrupole coupling constants, transfer of the HNO3 proton to its nearest water molecule is about one-third complete in the trihydrate.