In this work, hydrogen (H) plasma treatment is implemented to dope indium gallium zinc oxide (InGaZnO), zinc oxide (ZnO), and indium oxide (In2O3) thin-film transistors (TFTs). We systematically analyze the active defect states inside these n-type metal oxides and reveal how they are impacted by H dopant incorporation, combining the device transfer characteristics (including the threshold voltage, subthreshold slope, and carrier mobility), the X-ray photoelectron spectra, and numerical and theoretical investigations. An increase of the field-effect mobility of these TFTs is mainly attributed to the decreased interface and bulk tail-distributed traps, after an appropriate amount of H dopants is incorporated. In ZnO, hydrogen exclusively acts as a shallow donor during the plasma treatment, while the zinc vacancies Zn(Vac) cannot be passivated by the H dopants as no improvement of the subthreshold slope (SS) is observed in the hydrogenated ZnO TFT. The H interstitials (Hi) incorporated into In2O3 are stable in the + charge state at equilibrium, then change into the − charge state as the Fermi level energy EF gets closer to the bottom of the conduction band. Due to the H insertion into an oxygen vacancy VO, the VOH complex (acting as an acceptor) is formed in InGaZnO with increased H plasma treatment duration, leading to the degraded SS. This paper clarifies the H dopants’ role and the different dominant defects inside the three types of TFTs, which may benefit systematic understanding and exploration of H dopant incorporation into InGaZnO, ZnO and In2O3 films for TFT improvement and optimization.