Resignation not accepted

Dear Sir/Ma'm, In my current organisation my employer wanted me to work for more than working hours and on weekends too. Although it was not a standard practice but due to this i have some health issues like headache, backache etc. Apart from this they are providing me task other than my skillsets. At present my company does not have any in house projects and we are working on POC's only. The POC I was assigned to has been completed and I got a very good opportunity from other company which also belongs to my skillsets. My companies notice period is of 3 months but i have to join new company in 1 month. I submitted my resignation to the employer clearly mentioning that currently i am neither on any client location nor working on any project so my release will not result in any loss to company as well as the health issues. But my employer denied to accept my resignation saying that i have to serve 3 months notice period. Can my employer force me to work inspite of any project assignment. Can i exit after 1 month notice period. Thanks