As the demand for high-speed and low-power electronics continues to grow, the quasi-delay-insensitive (QDI) asynchronous domain of digital design has emerged as a promising alternative to traditional clock-based designs. However, the adoption of the paradigm has been greatly limited due to the lack of mature computer-aided design (CAD) tools and a substantially larger area footprint, owing to various architectural constraints. Monolithic-3D (M3D) technology has recently paved the way for manufacturing highly dense integrated circuits (ICs) through sequential integration, resulting in reduced footprint area, shorter wirelengths, and increased performance. In this study, we integrate M3D technology with QDI Null Convention Logic (NCL) and propose a design methodology for the implementation of M3D-based NCL standard cells, aimed at mitigating the area inefficiencies of traditional planar or 2D counterparts. Furthermore, we employed the threshold gates to design an M3D-NCL unsigned array multiplier circuit. Simulation results suggest that, for a conservative wirelength reduction resulting from M3D implementation, a substantial area reduction of 44% can be achieved while simultaneously reducing delay and power by ~31%, and ~17%, respectively.