Appointment Letter vs Consent Form

Hello everyone, I have worked at a private IT company for more than 3 years. I have resigned from the company and served notice period of 3 months as specified in Appointment Letter. However, my resignation was not accepted and I was asked to continue for another 3 months, to which I declined. As of now, I have stopped reporting to office since 3 months notice period has been completed. However, my resignation (sent via Email) still has not been accepted. Also, the company has refused to issue me with relieving letter. I SUSPECT that when I had joined the company, I may have signed a form in which I consented to extend my notice period by 3 months. This is just my my SUSPICION as I do not possess any copy of such form with myself. I do possess copy of my Appointment Letter(A Non Judicial Paper), Confirmation Letter and Increment Letter, which all state my notice period to be of 3 months only. Thus, I'd like to know, in case of dispute, which document should hold true? My Appointment Letter or the consent form?