EV3RT is, to our knowledge, the first RTOS-based software platform for LEGO Mindstorms EV3 robotics kit. It is faster and more suitable for developing applications with real-time requirements than other existing software platforms. In practice, EV3RT has been selected as one of the officially supported platforms of ET Robocon, a popular robot competition in Japan, since 2015 and used by many participating teams to make their robots accomplish the assigned tasks more stably and precisely. In this paper, the usage and architecture of EV3RT are firstly introduced. We then explain TOPPERS/HRP2 kernel, the RTOS of EV3RT, and how to use its protection functionalities to build a reliable platform. A mechanism to support dynamic module loading in a static RTOS is proposed to implement the application loader for EV3RT. Implementation techniques like approach to reusing Linux device drivers are also described. Finally, the advantages of EV3RT are shown by evaluating and comparing its performance with other platforms.