Group 1 is (roughly) the top third of the waiting list. There is no guarantee, but based on outcomes in recent years, there is a reasonably good probability of acceptance for applicants in this group.
Group 2 is slightly larger than Group 1. In an "average" year (statistically average), Group 2 applicants have a realistic possibility of acceptance. Again, acceptance is not guaranteed, but applicants in this group should expect to know more about their status by the second week of June.
Group 3 is the least predictable group. About half of the time we reach Group 3, and about half of the time we don't, and in some years we actually progress nicely into the group. In short, Group 3 outcomes vary considerably from year to year, but the situation is not completely unrealistic for applicants in this group, and whatever happens, we should know where we stand by the third week of June.