They have been sentenced to life imprisonment by the court.
Additional District and Sessions Judge B P Verma held the trio guilty under provisions of section 124 IPC (sedition), 120 B IPC (conspiracy) and Chhattisgarh Special Public Security Act.
58-year-old Sen, a paediatrician by training, had been accused by the prosecution of carrying messages and letters from Sanyal to the underground Maoists. Guha was also found involved in helping the Maoists establish a network.
The Chhattisgarh police had claimed that Sen was helping Naxals establish a network in urban areas.
Binayak, a PUCL activist, was arrested on May 14, 2007 from Bilaspur. He was present in the court along with Sanyal and Guha as the judge delivered the verdict. Sen's lawyer said that he will appeal the conviction.
Sen, who was freed on bail last year, has always maintained his innocence. "I have never condoned Maoist violence. It is an invalid and unsustainable movement," Dr Sen had said after his arrest in 2007.
The verdict has evoked strong reactions from activists as well as Dr Sen's family. His wife Elina Sen said that she was sad for the "state of Indian democracy".