Increase in notice period by employer over mail - what is actual enforceable notice period on me?

I am in situation. My organisation is a small organisation and I have signed one month notice period with them. But one fine day all employees received a mail from HR that now onwards all the employees has to give 3 month notice, if any one has any concerns reply to this mail.
Now what is actual enforceable notice period on me. By fear every one is giving 3 month notice now otherwise they are not giving full and final amount (according to policy they only give half salary in notice period) and not giving experience letter.
Please help with suggestion. Even it is difficult for us to fight big legal cases with employer.

Dear Friend,
In this case,what clause mentioned in your appointment letter during your joining so that will be admissible rather mail document.
I Suggest you to discuss with your HR mentioning your appointment letter clause, during your joining a letter signed between you and management, Inspite of this, they do their own arbitrators then please write a mail to management that you will go for legel help if management doesn't help regard this issue.