Neuromorphic computing utilizes spiking neural networks (SNNs) to offer power/energyefficient solutions for complex machine-learning problems in hardware. However, neural circuits are prone to faults caused by variability in the manufacturing flow, process variations, and manufacturing defects. This work proposes a mapping approach, R-MaS3N, that leverages the reuse of existing neurons for robust mapping of SNNs to a 3D-NoC-based neuromorphic system (NR-NASH). A heuristic-based partitioning technique is employed to partition neurons in the layers of an SNN application using neuron firing patterns. Moreover, a neuronal partitioning approach cluster mapped neurons in the neuromorphic neural circuits based on connectivity patterns and spiking activities. Evaluation results show that the proposed fault-tolerant mapping method maintains a remapping efficiency of 100% with a fault rate of 40% in the 3D NoC-based neuromorphic system. With a system configuration of 4 × 4 × 4 and 256 neurons per cluster, our approach has a remapping time of 71× less than the previous approach with the same configuration parameters. In addition, the MTTF of the mapping method for system configuration 5 × 5 × 5 network size at a 40% fault rate surpasses the previous method at 20% fault rate by 16% for 4 × 4 × 4 network size.