THE presidents of Zimbabwe, the Democratic Republic of Congo and Namibia will hold a summit in Harare on Sunday. A first meeting is to take place very early in the morning between Zimbabwe President Robert Mugabe and Namibian President Sam Nujoma, a diplomat said, without indicating when the two other presidents will arrive. Namibia, Angola and Zimbabwe are allies of DRC President Laurent Kabila against an armed rebellion that erupted in August 1998 with the backing of Rwanda and Uganda.