About releaving order

Last year I got an assistant professorship offer from Saveetha University, Chennai. In the offer letter, they mentioned 2 year probation period and 1 month notice period and also mentioned that I could leave college only after the academic year ( after May). They did not mention the salary, but they told me to give 75000/-over the phone call (I have a call recording). I had also some other offers too which were providing the same salary. However, I opted for Saveetha University and rejected others' offers. When I came here, they gave me an appointment order, which mentioned a 3-month notice period instead of 1 month and they had offered me 65000/-. As I had already rejected another offer, I had to join. They kept my certificates with them. I talked to HR, he told me that if I give 3 months' notice then he will provide a relieving letter. After 1 year and 3 months, I resigned (with a 3-month notice) on October 10 and met the principal, and now they are telling me they will only give my certificate and will not provide relieving orders and experience certificates. Rest they will give only after the academic year. What should I do?