How do groups of 3 people establish common ground in problem-solving interactions differently from groups of 2? Do they do this differently when they are supported by videoconferencing technologies? The laboratory experiment reported here manipulated group size and video mediation for participants carrying out a laboratory map task that allowed clear measurement of who knew what. In general, patterns of prior knowledge had much more impact than the number of speakers involved. Lengthier stretches of talk occurred particularly where knowledge was distributed between 2 of the 3 individuals; it was especially challenging for groups of 3 partially knowledgable participants to establish common ground. Participants seemed reluctant to adopt the role of side participants in the problem-solving discussions. The problem-solving interactions were surprisingly similar in face-to-face and high quality video-mediated conditions, even though the collaborations involved participants several hundreds of miles apart.