This book constitutes the refereed post-conference proceedings of the 3rd EAI International Conference on Sustainable Energy for Smart Cities, SESC 2021, held in November 2021. The conference was framed within the 7th Annual Smart City 360° Summit. Due to COVID-19 pandemic the conferences were held virtually. The 13 revised full papers were carefully reviewed and selected from 28 submissions. They present multidisciplinary scientific results toward answering the complex technological problems of emergent Smart Cities. The subjects related to sustainable energy, framed with the scope of smart cities and addressed along with the SESC 2021 conference, are crucial to guarantee an equilibrium among economic growth and environmental sustainability, as well as to contribute to reducing the impact of climate change.