The paper reports the experimental observation of a third-order exceptional line (EL) in a nitrogen-vacancy (NV) spin system at the atomic scale. By introducing multiple symmetries, the researchers successfully realized the emergence of the third-order EL with a single electron spin of an NV center in diamond. The behaviors of the exceptional points (EPs) under different symmetries were systematically investigated, demonstrating that symmetries play a crucial role in the occurrence of high-order EPs and related geometries. The work opens new avenues for exploring high-order EP-related topological physics at the atomic scale and potential applications in quantum technologies. The experimental setup and methods are detailed, including the realization of the non-Hermitian Hamiltonian and the measurement of the EPs and their degeneracies. The results show that the third-order EL can be observed when both parity-time (PT) symmetry and pseudo-chirality are introduced, while the EP structure reduces to isolated EPs when only PT symmetry is present, and disappears when both symmetries are broken.The paper reports the experimental observation of a third-order exceptional line (EL) in a nitrogen-vacancy (NV) spin system at the atomic scale. By introducing multiple symmetries, the researchers successfully realized the emergence of the third-order EL with a single electron spin of an NV center in diamond. The behaviors of the exceptional points (EPs) under different symmetries were systematically investigated, demonstrating that symmetries play a crucial role in the occurrence of high-order EPs and related geometries. The work opens new avenues for exploring high-order EP-related topological physics at the atomic scale and potential applications in quantum technologies. The experimental setup and methods are detailed, including the realization of the non-Hermitian Hamiltonian and the measurement of the EPs and their degeneracies. The results show that the third-order EL can be observed when both parity-time (PT) symmetry and pseudo-chirality are introduced, while the EP structure reduces to isolated EPs when only PT symmetry is present, and disappears when both symmetries are broken.