Forcing the employee to serve notice period

I am working in a s/w company and I have got a central govt of India PSU job. The govt PSU have asked me to join within 20 days from the date of offer of appointment. But my company insisted on 3 months notice, I 've requested for extension of joining date from the HR officials in PSU and they accepted to give 1 month extension.But the s/w company still insists on serving 3 months notice. HR official in PSU say that If i did not join the PSU my offer of appointment will be given to waiting list candidates. I have already served 1 month notice period and I daily keep asking for early release from HR officials in S/w compny but in vain. Even they are not ready to agree for notice buy out. I am bench resource with no work assignments. My resignation is already accepted by my Manager and HR and only relieving formalities pending.Still the HR insist to serve for 3 months notice. I have mentioned that I am working in a s/w company during intrvw so the PSU is asking for reliving order from the company to have a clean sheet against me. Please provide me suggestion and legal advise to this problem.