District and Sessions Judge Pongiappan also slapped a fine of Rs 1,000 each on all of them.
One woman gang member was acquitted in the case.
The sentences will run concurrently.
The prosecution case was that an eight-member gang had kidnapped Dr Baskaran on April 30, 2009 when he was on his way to play tennis near his house, later demanded Rs one crore ransom from his family and on failing to get it, killed him and disposed of his body in the Aliyar dam.
The case was transferred to CB-CID on June 20, 2009 after local police failed to make headway in the case.