An NYPD cop tracked his wife and her young new lover to a hotel — then fatally shot the man before killing himself in the horrific love triangle gone awry, law enforcement sources told The Post on Monday. Crazed off-duty Officer Sean Armstead, 36, had followed spouse Alexandra Vanderheyden, 35, and her 20-year-old beau, Edward Wilkins, to a La Quinta Inn in upstate Wallkill around 10:30 p.m. Sunday, sources said. Vanderheyden told investigators she believes that Armstead tracked her through her cellphone, sources said.