Two British soldiers and a Roman Catholic man have been shot to death, the first fatalities blamed on sectarian terrorists in Northern Ireland in 1988. Police said Capt. Timothy David Armstrong, 29, a member of the Ulster Defense Regiment, a locally recruited British army unit, was killed on a Belfast street. A second member of the regiment, William J. Stewart, 23, was fatally wounded near Coalisland, 35 miles west of Belfast. No group claimed responsibility for the killings, but it appears the soldiers were slain by guerrillas of the outlawed Irish Republican Army. Protestants extremists are believed to be responsible for the killing of a 19-year-old Roman Catholic, Billy Kane, in his Belfast home.