A teen charged in the botched mugging-turned-murder of Barnard student Tessa Majors is expected to plead guilty Tuesday to all charges in the indictment, The Post has learned. Luchiano Lewis, 16, one of three teens charged in the slaying of the 18-year-old freshman in 2019 in Morningside Park, has agreed to cop to second-degree murder, first-degree robbery and second-degree robbery, sources said. He’s cooperating with Manhattan prosecutors against co-defendant Rashaun Weaver, 16, who faces the same counts. The agreement is not part of a plea deal, sources said. Majors’ heartbroken family is expected to be in court for the significant development in the case, according to sources. Zyairr Davis, who was 13 at the time of the murder, was tried as a juvenile. He pleaded guilty to one count of first-degree robbery in 2020 and was sentenced to 18 months in detention. Prosecutors allege that Weaver and Lewis, who were 14 at the time, and Davis went to the park Dec. 11 looking to rob someone and spotted the Charlottesville, Virginia, native. After she screamed for help and tried to fight off her attackers, Lewis allegedly held her in a headlock and Weaver repeatedly stabbed her, sending the feathers in her down coat flying into the air, according to court papers. Before they set their sights on Majors, Davis said he handed Weaver the knife that was used in the murder, according to his videotaped confession. Justice Robert Mandelbaum previously ruled that Weaver and Lewis would be tried separately if their cases went to trial. Weaver’s attorney, Jeffrey Lichtman, declined to comment. Lichtman, who represented Mexican drug lord Joaquin “El Chapo” Guzman, agreed to defend Weaver for free, saying he didn’t deserve to face life in prison because of his age. Before Weaver was arrested for Majors’ murder, he allegedly committed another violent robbery Feb. 14, 2020. He and three pals brutally beat an unnamed man before stealing his cellphone, $130 and his shoes, court papers state.