The State Department formally added three more United Nations Relief and Works Agency school teachers to a first-of-its-kind blacklist after an ongoing U.S. investigation concluded that the teachers participated in Hamas's Oct. 7 terror spree and held Israelis captive, the Washington Free Beacon can reveal. It's the latest embarrassment for the United Nations, which has delegated most of its Gaza relief work to UNRWA even though the agency is fu…