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.

Chapters "Identifying Political Sentiments on YouTube: A Systematic Comparison regarding the Accuracy of Recurrent Neural Network and Machine Learning Models", "Do Online Trolling Strategies Differ in Political and Interest Forums: Early Results" and "Students Assessing Digital News and Misinformation" are available open access under a Creative Commons Attribution 4.0 International License via link.springer.com.