The development of novel nanozymes for environmental contamination remediation is a worthwhile research direction. However, most of the reported nanozymes cannot degrade efficiently due to the limitation of the internal active sites not being able to come into direct contact with contaminants. Therefore, we reported Fe-N-C single-atom nanozymes (SAzymes) with atomically dispersed FeN4 active sites anchored on a three-dimensional hierarchically ordered microporous-mesoporous-macroporous nitrogen doped carbon matrix (3DOM Fe-N-C) for the degradation of a targeted environmental pollutant (rhodamine B (RhB)). The three-dimensional (3D) hierarchically ordered porous structure may accelerate mass transfer and improve the accessibility of active sites. This structure and high metal atom utilization endow Fe-N-C SAzyme with enhanced tri-enzyme-mimic activities, comprising oxidase-mimic, peroxidase-mimic, and catalase-mimic activities. Based on its excellent peroxidase-mimic activity, 3DOM Fe-N-C can degrade RhB by hydroxyl radicals (·OH) generated in the presence of hydrogen peroxide. This study provides a new idea for designing porous Fe-N-C SAzymes for environmental contamination remediation.