This is Volume I of the proceedings of the 3rd European Conference on Argumentation: Reason to Dissent, held at the University of Groningen in June 2019. The European Conference on Argumentation (ECA) is a pan-European biennial initiative aiming to consolidate and advance various strands of research on argumentation and reasoning by gathering scholars from a range of disciplines such as philosophy, communication, linguistics, computer science, cognitive science, discourse analysis, and more. The 2019 Groningen edition focused on dissent. The goal was to inquire into the virtues and vices of dissent, criticism, disagreement, objections, and controversy in light of legitimizing policy decisions, justifying beliefs, proving theorems, defending standpoints, or strengthening informed consent. The papers in these two volumes discuss these and other questions pertaining to argumentation and dissent (among other themes). They can be regarded as a reflection of the current state of the art in argumentation scholarship pertaining to dissent and disagreement, and thus should be of interest to a wide range of scholars in different disciplines.