I need a query regarding notice period in job

hi, I had received offer letter from pvt college in kerala which didnt contain anything about notice period in it. I joined college and there they made to sign an appointment order which included notice period of 3 months. I was offered job till 70 years of age. during Covid time , chairman of college suddenly announced that he is handing over college to kerala govt because of financial issues in July 2020. And even govt team came to inspection of hospital and decision about was pending After that. Basically during that time, the HR of college wouldn't have guaranteed our job , since it becomes govt everything changes and basically since I am from karnataka , I may not be employed permanently in keral govt. So during that time , college HR was not sure about his job as well, and he couldn't have guaranteed that we will be working under that Pvt institutions , and we couldn't have got job till 70 years if it becomes government. And during this period, they didn't pay full salary which they had promised before. and now also not paying full. On 29th december 2020 I resigned as I was unsure about my job. On 6th Jan 2021, kerala government in parliament announced that it's not taking over college. now college is asking me to serve 3 months notice period. Shall I challenge it legally, because when I resigned , HR weren't even sure about their job. So how can they apply terms agreed ?? Kindly help, I am even ready to pay for online consultation. thank you.