A 19-year-old man was fatally shot by a romantic rival in East Harlem, police said Thursday. Ramon Alvarez was inside his apartment on E. 112th St. near Third Ave. — part of NYCHA’s Jefferson Houses — around 3:30 a.m. Wednesday when a gunman, who was romantically involved with the victim’s girlfriend, opened fire, hitting him in the chest. “They were banging on my door telling me someone had ...