JAKARTA, Indonesia—An explosive eruption Friday of Mount Dukono on the remote Indonesian island Halmahera killed three hikers and injured five who were climbing in a restricted area around the active volcano, authorities said. About 20 climbers had set out Thursday to ascend the nearly 1,355-meter (4,445-foot) volcano in defiance of safety restrictions, said North Halmahera police chief Erlichson Pasaribu. They became stranded when Dukono erupted at 7:41 a.m. local time, spewing a thick ash column that rose about 10 kilometers (6 miles) above the summit. The eruption was recorded on seismographs for more than 16 minutes, Indonesia’s Geological Agency said. “They were aware that climbing was prohibited as the mountain is a restricted zone due to its high alert status, but insisted on going ahead,” Pasaribu said in a television interview....