Two other accused in the case were acquitted.
Additional City and Sessions Judge Sanchita Kar pronounced Saiful Ali, Ansar Ali and Aminul Ali guilty of gangrape and murder in her order in a packed court room.
The three were found guilty under sections 376(d) (gangrape), 302 (murder) and 120B (criminal conspiracy) of IPC.
The judge found Imanul Islam, Aminul Islam and Bhola Naskar guilty under sections 376 (d) (gangrape), 120B (criminal conspiracy) and 201 (causing disappearance of evidence) of IPC.
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The judge said that the argument on the quantum of punishment would commence tomorrow.
While Saiful Ali, Ansar Ali and Aminul Ali may be given a minimum sentence of 20 years or a maximum punishment of death penalty, the other three found guilty may face a minimum of 20 years in prison or a maximum of life sentence, the judge said.
Rafiqul Islam and Nur Ali were acquitted owing to lack of evidence against them.
The gangrape and brutal murder of the 21-year-old college girl occurred when she was returning home at Kamduni, about 50km from Kolkata, in North 24-Parganas district after appearing for an examination at her college on June 7, 2013.
The incident had led to an outrage in the state amidst concerns over women's safety.
The second year BA student was pulled into a farm when she was walking back home along a deserted road after alighting from a bus.
She was gangraped and gruesomely murdered subsequently.
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Ansar Ali, one of the six convicted in the case, was today seen threatening the victim's brother outside the court room.
A couple of women, reported to be relatives of Ali, were also seen threatening the youth as he stood outside the court room.
Ali was whisked away by the police to the court lock up.
Members of several human rights groups were also seen shouting slogans in front of the court premises seeking harshest punishment for the accused since morning. Some even tried to enter the court building but were prevented by security personnel.