Regarding notice period.

Hello, I am employed with the company for past 2 yrs. I have only received offer letter from the company and not the appointment letter. We have asked company about the letter but haven't received one. Now the main prob starts here. In my offer letter there is a clause as under "Your employment will have a probation of 3 months from the date of joining. If your employment was terminated after probation period you will be given 3 months notice." Now I have got a new job . I have given the company one week notice period and have asked them to relieve me on 18. Now they are asking me to serve a notice of 3 months. My understanding of above clause is that 'if company terminates my enployment then i will be given a 3 months notice' correct me if i am wrong. Now They are arguing that the above clause can be interpreted in both ways. I.e even if I leave the company I will have to serve 3 months notice. Can they do that? What are my options regarding this. I have a mail from my manager saying I accept your resignation but according to company policy you have to serve 3 months notice period. Kindly advice .