Betrayed by their former boss — a South African crime lord who testified against them — three American men who worked as soldiers of fortune were convicted on Wednesday of a murder conspiracy plot to assassinate a real estate agent in the Philippines six years ago.

After only two hours of deliberations, a jury in Federal District Court in Manhattan returned a guilty verdict against the men, Joseph Hunter, Adam Samia and Carl D. Stillwell.

The government’s case against the defendants — presented at a spellbinding three-week federal trial — was built on the testimony of Paul Le Roux, a murderous kingpin who trafficked in guns, drugs and gold, and employed a private army of mercenaries on four continents to protect his assets and kill on his behalf. After Mr. Le Roux, 45, was arrested by the Drug Enforcement Administration in Liberia in late 2012, he began to cooperate with the government as they rounded up and prosecuted the members of his sprawling organization, in an effort to reduce a possible life sentence.

Mr. Hunter, a former United States Army sergeant with Special Forces training, was among the first to be caught. In 2013, Mr. Le Roux helped the D.E.A. set up a sting operation in Phuket, Thailand, during which they arrested Mr. Hunter, 52, for conspiring to kill one of their agents and a federal informant. Because the charges in that case included a plot to bring cocaine to New York, it was handled by prosecutors in Manhattan, who are also handling the murder in the Philippines, on the grounds it evolved from the same investigation.