We report on the electronic states and the transport properties of three-dimensional topological insulator (Bi<sub>1-<i>x</i></sub>Sb<sub><i>x</i></sub>)<sub>2</sub>Se<sub>3</sub> ternary alloy thin films grown on an isostructural Bi<sub>2</sub>Se<sub>3</sub> buffer layer on InP substrates. By angle-resolved photoemission spectroscopy, we clearly detected Dirac surface states with a large bulk band gap of 0.2 - 0.3 eV in the (Bi<sub>1-<i>x</i></sub>Sb<sub><i>x</i></sub>)<sub>2</sub>Se<sub>3</sub> film with <i>x</i> = 0.70. In addition, we observed by Hall effect measurements that the dominant charge carrier converts from electron (n-type) to hole (p-type) at around <i>x</i> = 0.7, indicating that the Fermi level can be controlled across the Dirac point. Indeed, the carrier transport was shown to be governed by Dirac surface state in 0.63 ≤ <i>x</i> ≤ 0.75.These features suggest that Fermi-level tunable (Bi<sub>1-<i>x</i></sub>Sb<sub><i>x</i></sub>)<sub>2</sub>Se<sub>3</sub>-based heterostructures provide a platform for extracting exotic topological phenomena.