This paper proposes an anisotropic three-dimensional (3-D) frequency-selective structure (FSS) design method, utilizing a variety of stacked slotlines and microstrip lines (MSL) based slow-wave structures, which are mapped by a generalized filtering network. Additionally, leveraging the phase slope relationships of adjacent- or different-order networks, this paper introduces a unified phase design concept for spatial wave manipulation, encompassing both in-band constant phase slopes and true-time delay characteristics. Using the numerical short-open-load calibration extraction, this paper thoroughly compares propagation performance and slow-wave coefficient between slow-wave structures and uniform transmission lines. Finally, the feasibility of the proposed design concept is verified by designing a right-hand to left-hand circular polarization converter mapped by orthogonal fourth- and sixth-order wideband filtering topologies. The simulation results agree with the theoretical prediction and have good angular stability.