Additional District Judge Narendra Kumar Singh also imposed a fine of Rs 20,000 each on the convicts.
Ten persons named in the FIR died during the course of the trial.
According to prosecution, Tribhuvan Dutt, his brother Baldev and nephew Chandra Prakash were killed on March 26, 1983 in Semerhaniya village.
The trio had been called on some pretext and beaten to death by the accused.
Baldev's son Dinanath had lodged an FIR against 25 persons the next day.