Terahertz (THz) communications are envisioned as a critical technology for 6G and enable ultra-fast dense networks with tiny cells. Implementing THz-enabled ultra-fast cells requires new techniques due to its 3D nature. Stochastic geometry is a tool that is widely used for network deployment, which provides tractability and feasibility. In this article, we provide a summary of 3D network modeling for the THz system. We discuss the new 6G ultra-cell scenario along with a potential 3D network architecture. Several recent developments in network modeling applying a stochastic geometry-based clustered process are also presented. We also discuss several potential network modeling techniques such as Poisson Voronoi (PV) tessellation, Thomas cluster process (TCP), and Matérn cluster process (MCP). Subsequently, a 3D network model for THz-band ultra-cell is demonstrated. Finally, we discuss major KPIs for 3D stochastic geometry and conclude with research challenges and open issues for 3D THz architectures.