The accused Sumit Bajaj, Ruman Khan and Naseer Khan were convicted by the court.
The sentence is expected to be delivered by the court on Friday.
"The accused have been convicted under 120 (B) conspiracy impersonations, 806, 323. I am happy for our victory and sorry for their conviction. They are tender-aged boys and I pray for their development," said Public Prosecutor Sarbani Roy.
"The court held them guilty. Quantum of punishment will be acquitted to them tomorrow at 2 p.m.," said Tapas Bhattacharya, the lawyer for the accused.
The victim, who had survived the gang-rape inside a moving car in Kolkata three years ago, died in March, 2015, due to multiple organ failure after she was diagnosed with encephalitis.
Five accused were identified by officials out of which only three were arrested. The remaining two, including the prime accused Kader Khan, are still absconding.
Expressing satisfaction over the judgment in the case, the victim's father said he is very happy for his family.
"I am sorry for their family. I am very happy for my family. I can't suggest any form of punishment for anybody. What is the use of talking about punishment now? My daughter is already dead," he said.
The victim's grandmother also expressed content.
"We got what she fought for. We are very happy. I am satisfied," she said.