This book constitutes the refereed proceedings of the Third Multidisciplinary International Symposium on Disinformation in Open Online Media, MISDOOM 2021, held in September 2021. The conference was held virtually due to the COVID-19 pandemic. The 9 full papers were carefully reviewed and selected from 27 submissions. The papers focus on health misinformation, hate speech, misinformation diffusion, news spreading behaviour and mitigation, harm-aware news recommender systems.