In the continuity casting technology of steel-manufacturing process, the tundish has two important functions: the preservation and distribution of molten metal and the reaction container able to perform float-out separation of nonmetallic inclusions. The residence time affects the effective removal of the nonmetallic inclusions. In this study, a T-shaped tundish with a submerged entry nozzle (SEN) and three strands was investigated for its ability to extend the residence time. Analysis conditions were the shape of the dam which was transformed to three cases. Fluid flow and non-metallic inclusion movement were also analyzed. The movement and removal of nonmetallic inclusions was determined by residence time distribution (RTD) analysis. As a result, the number of float-out, non-metallic inclusions was increased when the deviation of mean residence time was reduced.