This paper introduces the third DIHARD challenge, the third in a series of speaker diarization challenges intended to improve the robustness of diarization systems to variation in recording equipment, noise conditions, and conversational domain. Speaker diarization is evaluated under two segmentation conditions (diarization from a reference speech segmentation vs. diarization from scratch) and 11 diverse domains. The domains span a range of recording conditions and interaction types, including read audiobooks, meeting speech, clinical interviews, web videos, and, for the first time, conversational telephone speech. We describe the task and metrics, challenge design, datasets, and baseline systems for speech speech activity detection and diarization.