This book constitutes the proceedings of the Third International Conference on Frontiers in Cyber Security, FCS 2020, held in Tianjin, China*, in November 2020. The 39 full papers along with the 10 short papers presented were carefully reviewed and selected from 143 submissions. The papers are organized in topical sections on: IoT security; artificial intelligence; blockchain; cyber-physical systems security; cryptography; database security; depth estimation; mobile security; network security; privacy; program analysis; quantum cryptography; steganography; web security.*The conference was held virtually due to the COVID-19 pandemic.