This book constitutes the refereed proceedings of the Third EAI International Conference on Smart Grid and Internet of Things, SGIoT 2019, held in TaiChung, Taiwan, in November  2019. The 10 papers presented were carefully reviewed and selected from 22 submissions and present results on how to achieve more efficient use of resources based largely on the IoT-based machine-to-machine (M2M) interactions of millions of smart meters and sensors in the smart grid specific communication networks such as home area networks, building area networks, and neighborhood area networks. The smart grid also encompasses IoT technologies, which monitor transmission lines, manage substations, integrate renewable energy generation. Through these technologies, the authorities can smartly identify outage problems, and intelligently schedule the power generation and delivery to the customers. Furthermore, the smart grid should teach us a valuable lesson that security must be designed in from the start of any IoT deployment.