This paper proposes a new sliding mode-based guidance law for mobile robots in the three-dimensional space that offer strong robustness properties and intuitive tuning. The guidance law is proven to guide a vehicle to a boundary region centered in the path particle in finite time. Furthermore, it is proven that for any state inside the boundary region, the guidance law is exponentially stable. Due to its stability properties, the proposed guidance law can guarantee a minimum rate of decay and thus estimate the time of convergence. The stability and robustness properties of the guidance law are analyzed through Lyapunov theory, cascaded system theory, and simulations.