Trending @Capgemini; Notice period issue

I got placed in a company by saying that I'm currently serving my notice period. But in reality, I haven’t started serving it yet — I still have a 90 days notice period. Now, how can I get an early release? I know I need to get approval from HR and my manager for early release, but in case that doesn’t happen, how can I explain this to the company that offered me the job?