NEW YORK — Three people have been arrested in the killing of a man on a New York City subway car last week, police said Tuesday.William Alvarez, a 45-year-old Bronx resident, was fatally shot after getting into an argument with one of the suspects on a D Train in the Bronx around 5 a.m. Friday. Police said what happened was captured on surveillance videos on the trains, subway station and on the street.The three have been charged with murder and criminal possession of a weapon. They were…