Three interviewers (I had a PhD prof, an MD, and a 4th year studend). Three interviewees, paired together apparently at random. I didn't think it was too stressful, but they did put one of the other applicants on the spot for a question or two, which was probably stressful considering she had two peers staring at her throughout. Overall, the idea is to build a cooperative mini-group during the interview. While the questions start out pointed and individual (it's a closed-file interview, btw), they progress towards more conversational all-inclusive questions, until you finally have to work through some question as a group and present your answer.
We had to come up with a new method of accepting/rejecting medical school applicants. Some other group had to assume the roles of FEMA, LA gov, and LA mayor and talk about the preventative and emergency plans they would each enact, and how they would cooperate (it was right after Katrina).
You'll have some time before the interview to talk with the two others you'll be with. Good time to find similarities and differences, and discuss how you'll tackle the group question.