This book constitutes the refereed proceedings of the Third Symposium of the Norwegian AI Society, NAIS 2019, held in Trondheim, Norway, in May, 2019. The 11 full papers and 3 short papers were carefully reviewed and selected from 21 submissions. The papers focus on all aspects of: artificial intelligence; machine learning; knowledge representation;  robotics; planning and scheduling; natural language processing; computer vision; search algorithms; multi-agent-systems; industrial applications; and philosophical and ethical foundations.