What can I do if company refuses to issue certificates?

I worked as an Assistant Professor in a private medical college. Within one year of probation, they terminated my job, stating that I went for a fellowship on May 14 without prior notice. However, I had received permission from the Dean and the Medical Superintendent on May 10. Now, I have received a "no dues" certificate signed by the Dean, MS, and Deputy Director, but the HR department is not issuing my experience and relieving certificates. They are insisting that I didn't serve the notice period and are asking me to pay for the 3-month notice period. They terminated me from my job; I did not resign.