Special court judge Gunasekharan said the sentences have to run consecutively.
The prosecution case was that the trio - Uday Kumar (19), S Paramasivan (23) and Syed Abdul Khadar (29) - had done to death powerloom unit owner Rakiappan and his daughter Vanitha in their house in Kumarapalayam on the night of December 24, 2014.
Kumar and Khadar were employed in the powerloom unit run by Rakiappan.
The three also killed Vanitha as she was witness to the murder.
The prosecution contended that the three also attempted to kill Rakiappan's wife and younger daughter, who rushed to the spot after hearing their screams. They, however, escaped with serious injuries.