Company threatening to ruin career - leavings because work promised to me during the interview is completely different from what is assigned

I've recently joined a company. I've found out that work promised to me during the interview is completely different from what is assigned to me.
I've talked to my supervisor and told him that I would like to leave as soon as possible.
My supervisor reverted back as follows "I've talked to the CTO and your notice period will be 2 weeks".
The next working day I got a mail from H.R mentioning that the notice period is 3 months as per the agreement.
Now they are threatening to take legal action. They are also saying that they'll ruin my career prospects by complaining against me at U.S embassy/consulate.
They aren't even open to payment of salary for the notice period.
How should I proceed? Please advice.

Dear member,

You have given a few details, however, your post is still vague. Please provide the following information?

a) Date on which you have joined the company?
b) Have you been issued the appointment letter? Please note the difference between the "Offer Letter" and the "Appointment Letter". The former is issued once the company is satisfied with the candidature of the candidate. The latter is issued once the candidate joins the company.
c) What is the notice period as per the appointment letter?
d) Where is your place of work? I mean in which city, state, country?
e) Is your employer located in India or somewhere else?

The above questions are asked to assess whether the jurisdiction of Indian labour laws is applicable or not.

Thanks,

Dinesh Divekar