NEW DELHI (Reuters) - A court sentenced three men to life in prison on Thursday for murdering a Sikh constable, his son and son-in-law during the 1984 anti-Sikh riots sparked by the assassination of then Prime Minister Indira Gandhi.

Nearly 3,000 people, mostly Sikhs, were killed in the riots in New Delhi in retaliation against Gandhi's murder by her Sikh bodyguards. But there have been few convictions.

Hans Raj Bhardwaj, 62, Ram Prakash Tiwari, 58, and Jagdish Giri, 65 -- all Hindus -- were part of a mob that killed constable Niranjan Singh and ransacked his house, the court said.

They returned a day later with swords to kill Singh's son and son-in-law in front of his widow, Harminder Kaur.

"The offenders of such a gory incident deserve no leniency," Y.P. Singh Ahluwalia, the state prosecutor, argued at the trial court in New Delhi on Thursday, while pushing for the death penalty.

After the men were sentenced to life, Ahluwalia said the prosecution would appeal to the high court for the death sentence. Two other men accused of involvement were acquitted.

Only two other anti-Sikh riot cases have resulted in convictions. Around 80 cases are pending.

Gandhi's assassination on Oct. 31, 1984 was carried out in revenge for her decision to send the army to flush Sikh separatists out of the Golden Temple -- Sikhism's holiest shrine -- in Amritsar in June 1984. The raid damaged the shrine, enraging Sikhs.

The government says nearly 3,000 Sikhs were killed in the riots following her death, while human rights activists say the figure is closer to 4,000.

Activists accuse the ruling Congress party of having turned a blind eye to the killing of Sikhs and say some of its leaders helped orchestrate the rioting.

Sikhs make up around two percent of Hindu-majority India's population of more than one billion.