This study reports the experimental observation of a third-order exceptional line (EL) in a nitrogen-vacancy (NV) spin system at the atomic scale. The third-order EL is formed by multiple high-order exceptional points (EPs) and is realized by introducing symmetries into a non-Hermitian Hamiltonian. The study demonstrates that symmetries play a crucial role in the emergence of high-order EPs and their associated geometries. The third-order EL is observed in a two-dimensional parameter space, and its properties are systematically investigated under different symmetry conditions. The results show that the third-order EL exhibits richer topological characteristics and enhanced performance in sensing compared to second-order EPs. The study also highlights the potential applications of high-order EPs in quantum technologies. The non-Hermitian Hamiltonian used in the experiment is designed to exhibit third-order EPs by introducing specific symmetries. The study provides a new avenue for exploring high-order EP-related topological physics at the atomic scale and demonstrates the feasibility of using NV centers as a platform for non-Hermitian quantum physics experiments. The results are supported by theoretical predictions and experimental data, showing good agreement between the two. The study also discusses the role of symmetries in the occurrence of high-order EPs and their geometries, and highlights the importance of symmetries in controlling the parameters of the non-Hermitian Hamiltonian. The study provides a comprehensive understanding of the behavior of high-order EPs and their potential applications in quantum technologies.This study reports the experimental observation of a third-order exceptional line (EL) in a nitrogen-vacancy (NV) spin system at the atomic scale. The third-order EL is formed by multiple high-order exceptional points (EPs) and is realized by introducing symmetries into a non-Hermitian Hamiltonian. The study demonstrates that symmetries play a crucial role in the emergence of high-order EPs and their associated geometries. The third-order EL is observed in a two-dimensional parameter space, and its properties are systematically investigated under different symmetry conditions. The results show that the third-order EL exhibits richer topological characteristics and enhanced performance in sensing compared to second-order EPs. The study also highlights the potential applications of high-order EPs in quantum technologies. The non-Hermitian Hamiltonian used in the experiment is designed to exhibit third-order EPs by introducing specific symmetries. The study provides a new avenue for exploring high-order EP-related topological physics at the atomic scale and demonstrates the feasibility of using NV centers as a platform for non-Hermitian quantum physics experiments. The results are supported by theoretical predictions and experimental data, showing good agreement between the two. The study also discusses the role of symmetries in the occurrence of high-order EPs and their geometries, and highlights the importance of symmetries in controlling the parameters of the non-Hermitian Hamiltonian. The study provides a comprehensive understanding of the behavior of high-order EPs and their potential applications in quantum technologies.