A three-stage gas gun, composed of a two-stage gas gun and the add-on part, has been developed to launch high-Z (tantalum, for example) flyer plates up to 10 km/s for ultrahigh-pressure Hugoniot measurements. Great care has been taken to optimize the add-on part in which a specially designed graded density impactor is employed to quasi-isentropically accelerate the high-Z flyer plate for maximizing its impact velocity. The shock wave in the target generated by the flyer plate is characterized with the flatness of the shock-front better than 1 ns in the concerned area and the uncertainty of the shock-wave velocity less than 2%, thus satisfying the requirements for high-precision Hugoniot measurements. As a demonstration, we measured the ultrahigh-pressure Hugoniot equation of state of tantalum ranging from 0.45 TPa to 0.85 TPa with a symmetric impacting geometry in which the shock-wave velocity and the particle velocity are simultaneously determined. The results obtained are well consistent with data available in the literature, indicating the extended capability of the gas-gun launcher technique.