We report a photoelectron spectroscopy and density functional theory (DFT) study on the electronic and structural properties of Nb-3(-), Nb3O-, Nb3O2-, and the corresponding neutrals. Well-resolved photoelectron spectra are obtained for the anion clusters at different photon energies and are compared with DFT calculations to elucidate their structures and chemical bonding. We find that Nb-3(-) possesses a C-2v((3)A(2)) structure, and Nb-3 is a scalene C-s ((2)A '') triangle. Both Nb3O- and Nb3O are found to have C-2v structures, in which the O atom bridges two Nb atoms in a Nb-3 triangle. The ground-state of Nb3O2- is found surprisingly to be a low symmetry C-1 ((1)A) structure, which contains a bridging and a terminal O atom. Molecular orbital analyses are carried out to understand the structures and bonding of the three clusters and provide insights into the sequential oxidation from Nb-3(-) to Nb3O2-. The terminal Nb=O unit is common in niobia catalysts, and the Nb3O2- cluster with a Nb=O unit may be viewed as a molecular model for the catalytic sites or the initial oxidation of a Nb surface.