THREE SMALL CHILDREN died early Saturday when fire apparently sparked by a cigarette swept through their home in Dover, N.H., leaving another youngster severely burned and injuring four adults, officials said. AN EARTHQUAKE measuring 6.2 on the Richter scale rocked a remote area of China Saturday, but there were no immediate reports on casualties or damage, the official Xinhua news agency reported. The quake struck the central portion of Gansu province in an area inhabited by Tibetan ethnic people, Xinhua said.