This paper details the development of the third version of the FloripaSat CubeSat platform, focusing on enhanced reliability and robustness. CubeSats have gained popularity due to their low cost and rapid development cycle, yet the affordability of commercial platforms remains a barrier for many educational and research institutions. The SpaceLab team at UFSC aims to address this issue by creating an open-source, robust satellite platform. The FloripaSat-3 platform improves upon its predecessors, FloripaSat-1 and 2, by introducing a systematic design methodology for key subsystems such as On-Board Data Handling (OBDH), Telemetry and Telecommand (TMTC), and the Electrical Power System (EPS). The design process is based on ECSS guidelines and divided into four stages: requirement analysis, schematic and layout design, preliminary testing, and final adjustments with full subsystem integration. The primary goal is to create a reliable platform that can be reused for various missions, addressing the limitations encountered in earlier versions, particularly those related to scalability and processing power. Additionally, the paper emphasizes the open-source nature of the project, providing detailed documentation, schematics, and block diagrams, which can be further developed by other teams. FloripaSat-3 is expected to serve as a foundation for more ambitious space projects, making space technology more accessible to a broader range of users and contributing to the evolution of small satellite platforms.