The sentences will run concurrently, district Mahila Court Judge P Thirunavukkarasu said yesterday.
The prosecution case was that the trio entered the house of the woman in Nochikkattuvalsu village, 10 km from here on July 2012 evening when she was alone and robbed her of gold bangles. After the woman raised an alarm, they stabbed her and fled.
The three were later arrested.
The judge yesterday sentenced the three to life imprisonment for murder.
The three were later taken to Coimbatore Central prison.