District assistant judge Nazima Banu also imposed a fine of Rs 15,000 each on Perumal (65), Kunjupaiyan (39) and Govindaraj (34) of Thalasampatti near here.
Prosecution case was that Palanivel, 35, had an illicit relationship with the wife of Govindaraj and when it came to light she committed suicide.
Later, the father and his two sons waylaid Palanivel and murdered him on November 10, 2011.