Have resigned from a company with immediatee effect

Hi, I have joined an Indian MNC two months back.During the interview process they never mentioned about any shifts.I got to know there will be rotational and night shifts after joining and spending few days in the company.I have serious health issues and i have gone thorough an operation few months back.Its not possible for me to work in shifts.I discussed about the problem with my Supervisor and informed that I want to resign from the company with immediate effect.He said I have to submit my medical documents and than they will decide if I need to serve entire 3 months notice.That was not possible for me to serve any notice and I dont need any experience letter from the company.I have dropped the resignation mail and medical documents to my Supervisor and HR and asked for immediate relieving and mentioned that as my last working day. Next day I have not reported to office.HR has sent me a mail that they were trying to reach me.They want to either serve 3 months notice or pay for the shortfall of notice period to make it a clear exit.Failure to which they will be constrained to close it as per policy. Please suggest the next course of action.Also please note,I dont need any experience letter from the company.