Permanent magnet (PM) electrical machine has far-reaching impacts in aviation electrification due to the continuous development in high power density and high efficiency electrical drives. The primary barrier to acceptance of permanent magnet machines for safetycritical starter-generator systems is its low fault-tolerance capability and low reliability (for the conventional designs). This article investigates a modular triple three-phase PM starter-generator comprehensively, including the tradeoff of fault-tolerant topology, optimization design process, analysis of electromagnetic (highlight the post-fault analysis) and thermal behavior, respectively. The triple three-phase segmented topology proposed meet the fault-tolerant requirement along with complete electrical, magnetic, and thermal isolation. There would be cost penalty on the proposed topology, but it gets offset by the ease of manufacturing of coils and their insertion. The stator gaps are introduced while doing segmentation which would provide unique opportunity to provide innovative cooling solutions. The multi-objectively optimization method is employed to search an optimal candidate of modular-designed machine. The detailed electromagnetic analysis of this candidate has been performed, while the post-fault performances have been highlighted, especially under the short-circuit scenarios. After that, the thermal behavior of this modular machine has been studied by the means of Computational Fluid Dynamics (CFD). By a parametric analysis, the varied size of segment gap seems no effect to the thermal management. Moreover, the housing water jacket cooling method is chosen to make the machine work safely. In all, this modulardesigned triple three-phase machine meet the requirement for an aircraft application with complete fault tolerance and increased reliability.