If the company really wants you, they can wait for 3 months, there was times when my previous company waited for 3 months just to employ a new employee. It might be because of the scarcity of the skillset that the person had. In my opinion, if the new company is willing to buy you out, it is always good to negotiate with them to let u serve at least 1-2 weeks (if it is urgent) notice so that you can handover your current tasks properly, this will create a good impression to the new employer that you are a responsible person and you will do you best to avoid disrupt the operation of a company.