The models of 3D unsteady coupled fluid-structure systems such as swimming fins are numerically too expensive to be optimized. Designers must therefore make preliminary decisions based on simplified models. In particular, considering 2D cases for optimization is a standard practice. The 2D design then needs to be translated into its 3D counterpart. The current work is an example of such a problem. The objective is to design a 3D swimming monofin using a 2D optimal flexural stiffness distribution determined in [1]. In this study, a 2D monofin was represented by rigid bars linked by torsional springs, in dynamic equilibrium with the fluid. The swimmer was composed of linear articulated segments, whose kinematics was imposed and identified from experimental data. The sheet vortex fluid model presented in [2] was used, which accounts for a two-dimensional unsteady, inviscid and incompressible fluid flow going past a thin obstacle. The propulsive power provided by the monofin has been maximized with an upper bound on the total power expended by the swimmer. The design variables were the spring rigidities.