Janet Jackson has postponed three more shows because of an undisclosed illness. Her publicist said in an e-mail late Saturday that Jackson was postponing a Saturday show in Greensboro, N.C., one on Sunday in Atlanta and a third on Tuesday in Fort Lauderdale. A statement from Jackson said she arrived in Greensboro, N.C., hoping to perform there Saturday, but a local doctor advised that she not perform after it became “evident” she was not fully recovered. Representatives for the 42-year-old singer say she became “suddenly ill” and was hospitalized Monday night in Montreal shortly after she arrived for a show. She also canceled concerts in Boston and Philadelphia on Wednesday and Thursday. Jackson’s publicist did not elaborate Saturday, only saying she was “recuperating.” The note said Jackson will return home, at her doctor’s direction, for further treatment. In the note Jackson asked people to keep their tickets for the postponed shows, and said the promoter is working to reschedule dates.