The inherently destructive nature of laser-induced breakdown spectroscopy (LIBS) precludes repeated sampling of the same surface region, rendering simultaneous multi-element acquisition essential for LIBS elemental mapping. In this study, a picosecond LIBS mapping system was integrated with a custom-developed sagittal-focused multiband spectrometer to achieve simultaneous multi-element two-dimensional (2D) and three-dimensional (3D) microanalysis of segregation in steel samples. The sagittal-focused optical design efficiently concentrates spectral signals onto the effective detection window of linear array detectors, enabling reliable single-shot spectral acquisition under the weak-signal conditions inherent to high-spatial-resolution mapping. Operating at a spatial resolution of 2 μm, the system simultaneously captured emission lines of four critical alloying elements, Al, Cu, Ti, and Mn, across two spectral channels in a single mapping scan. For 3D microanalysis, a defocus-focused coordinated scanning strategy enabled 12-layer depth-resolved elemental imaging within a volume of 100 × 100 × 24 μm3 with stable signal quality across all layers. These results demonstrate that integrating sagittal-focused multiband detection with picosecond LIBS mapping provides a comprehensive and practically irreplaceable tool for simultaneous, multi-element microstructural characterization of steel segregation.