Electronic and structural properties of a series of tri-tungsten oxide clusters, W3On- and W3On (n = 7-10), are investigated using photoelectron spectroscopy and density functional theory (DFT) calculations. Both W 5d and O 2p detachment features are observed for n = 7-9, whereas only detachment features from O 2p-type orbitals are observed for W3O10- at high electron binding energies (> 7 eV). A large energy gap (similar to 3.4 eV) is observed for the stoichiometric W3O9 cluster, which already reaches the bulk value, suggesting that W3O9 can be viewed as the smallest molecular model for bulk WO3. DFT calculations are carried out to locate the most stable structures for both the anion and neutral clusters; time-dependent DFT method is used to predict the vertical detachment energies and to compare with the experimental data. It is shown that W3O9 possesses a D-3h structure, in which each W atom is tetrahedrally coordinated with two bridging O atoms and two terminal O atoms. W3O8 and W3O7 can be viewed as removing one and two terminal O atoms from W3O9, respectively, whereas W3O10 can be viewed as replacing a terminal O in W3O9 by a peroxo O-2 unit. We show that W3O8 contains a localized W4+ site, which can readily react with O-2 to form the W3O10 clusters with a calculated O-2 adsorption energy of -78 kcal/mol. It is suggested that the W3O8 cluster can be viewed as a molecular model for O-deficient site in tungsten oxides.