Hi Harry,
I feel sorry about your situation.
This is well known fact that, if there is a conflict between the employee and the manager, the manager will use his/her authority when time comes to hurt or take revenge on the employees.
This is really a culture problem and there should be open door policy where you can contact higher hierarchy to sort it out. These are organizational issues.
Now, for your issue apart from legal side ( as I see less chance that your appointment has the details of 3 months notice and you signed it), the only option available as far am concerned is talk to the manager, be humble and ask for a quick talk.
Apologies for whatever happened, and inform that it's career and life and seek help hopefully this may work. Also I suggest, you to talk to well known person of the manager in the same office. He/She may convince your manager and help you.
The ball is in your court, only you know about your manager and for sure there will be a way to get this done.
In worst case, you have to convince your new employer for revised dates stating some critical requirements. It's you, who need to break this huddle.
Relax, take sometime, write down all the ways to get this done and try. You will be able to fix this.
Good luck ,
Regards
Thiyagu