notice period changes

hi, my name is Satya I am working in one Pvt bank(recently the company got license) when I joined that company it is a financial services company but now it's a bank
when I got a offer letter last 1.5 year bank in that it is shows that the notice period is 1month last 10 days back they changed it to 3months I resigned yesterday because of some problems now they are saying me as per company rules you have to do 3months notice period compulsory
that to be on other place what should I do