A turbine, a gas turbine, and a method of disassembling turbine blades, capable of ensuring stable sealing performance as well as facilitating disassembly of turbine blades are provided. The turbine may include a rotor disk including a plurality of slots formed therein, a plurality of turbine blades spaced apart from each other by a predetermined distance in a circumferential direction of the rotor disk on an outer peripheral surface, each of the turbine blades being inserted into an associated one of the slots, and a retainer sealing a cooling passage defined between the rotor disk and the turbine blade, wherein the turbine blade includes an airfoil-shaped blade part, a root part inserted into the rotor disk, and a platform part located between the blade part and the root part, and the root part includes a blade hook protruding therefrom to support the retainer.