In this paper, a multi-objective design exploration for a three-element airfoil which consists of a slat, a main wing, and a flap was carried out by paying attention to the span wise sweep angle effect for a civil aircraft. Reynolds Averaged Navier-Stokes Solver (RANS) was used for the evaluation during the design process. In this study, sweep angle effect is focused, and "2.5-dimensional span wise design" was developed. In this calculation, two same planes were arranged along span wise direction. They were diagonally arranged to represent the sweep angle and periodic boundary condition was used for the simplified 2.5 dimensional span wise calculation. Kriging based Multi-objective Genetic Algorithm (MOGA) called 'efficient global optimization (EGO)' and Analysis of Variance (ANOVA) were used for the design exploration. The objective functions were defined as the maximization of lift coefficient at landing and near stall conditions simultaneously. 90 sample points were evaluated for the construction of the Kriging model. Based on present evaluation method, the span wise flow was observed and it has an influence on the separation on the wing. Through the design exploration process, the differences of the designed results between 2.5D and 2D evaluation were observed by visualizing the design space. It was also observed that the span wise flow influences the separation point and suction peak on the wing elements.