Ring segment, and turbine and gas turbine including the same

A ring segment, a turbine, and a gas turbine having improved cooling performance are provided. The ring segment may include a shielding wall mounted to a turbine casing which accommodates a turbine blade and configured to face an inner circumferential surface of the turbine casing, a first hook part and a second hook part configured to protrude from the shielding wall toward the turbine casing to be coupled to the turbine casing, a main cavity formed between the first hook part and the second hook part, a plurality of first cooling passages configured to connect the main cavity and first side surfaces facing each other of the shielding wall, a plurality of second cooling passages configured to extend in a direction crossing the first cooling passage and connect the main cavity and second side surfaces facing each other of the shielding wall, and a chamber configured to be connected to the first cooling passages.