Once they have entered into a contract providing for 1 month notice period, they cannot afterwards unilaterally change it to 3 months - any change/variation/novation(substitution of old contract with new one) can be done only with consent of both you and employer. This particular term is not binding on you and is as good as none, your silence even cannot be taken as implied admission of the term. 

However my opinion is still subject to a perusal of the email and original contract.