Connectivity is an important aspect of wireless sensor networks. Providing connectivity in 3D wireless sensor networks is very crucial when the network size is larger. There are several studies that discuss connectivity in the 2D sensor network. However, these are not always suitable when the network area is non-planar. In this paper, we focus on the connectivity of sensor networks in 3D space. We propose a Delaunay tetrahedron-based relay node placement strategy for connectivity and evaluate sensor density for the requirement. Sensor networks on the mountain are a form of a 3D surface topography. The target region is triangulated using the Delaunay triangle. Reduction in hops for data forwarding consumes less energy. Using such techniques, we developed an energy-efficient algorithm. Through simulation, we demonstrate that the proposed method guarantees connectivity with an optimal number of sensor nodes.