Company asking me to serve 3 months notice period

My organisation gave me an offer letter stating one month as the notice period upon confirmation. Then they gave us another offer letter within the same month saying 3 months notice period to be served upon "confirmation". The confirmation was to be done after 6 months of employment. After repeated mails on completion of 6 months, the company didn't confirm me and also verbally extended my probation period. I resigned after serving for 11 months (still being on the probation). the company now is asking me to serve a 3 months notice. My question is when they didn't confirm me and extended my probation how can they ask me to serve for three months notice. the company is not reliving me. what should i do?