If you fail in 3 subjects then in general, you won't be promoted to the next class i.e., your 12th. Now this depends on your board as well as your school. From a general perspective, failing in more than 2 subjects is considered to be Failing the whole session.

I would recommend you to contact your class teacher/school authorities & they will guide you out. Sometimes, on the basis of Guardian calls & assurance from the guardians the students are promoted but again its dependent on your school.

I think this answer provides you some insight regarding your problem. Have a great eve!