In a seal member, a turbine, and a gas turbine, a plurality of vanes includes a vane main body extending in a radial direction of an axis, an outer shroud, an inner shroud and forming a main flow path of a combustion gas between the outer shrouds and the inner shroud, and a retainer protruding from an inner surface in the radial direction of the inner shroud to the inside in the radial direction. The retainers in the vanes, which are adjacent to each other in the circumferential direction, are adjacent to each other in the circumferential direction via a third gap, and relative to one end of the seal member, the other end is positioned downstream in a direction along the axis in the combustion gas to close the third gap.