Service tax not paid for last three years?

Dear Frieds,

One of my client had not paid service tax for the last three year i.e from FY 2012-13 to till date and now he want to clear all his dues by paying the service tax along with interest etc/.please let me how to calculate the interest for the last three years .Turn over for FY 2012-13 is Rs 500000/- only where as TO for Rs 2013-14 is Rs 60L and for FY 2014-15 till date it is Rs 1.0 Crore.please let me how to arrive with the total tax payment ?

Thanks 

Ankit