Hi ,
If you are in 12 start preparing from now onwards and your concepts should be clear in all subjects as it will build your base for solving much tougher questions. And do not go for only subject to clear cutoff ,  give  equal time to all three subjects and revise them properly . At the time of 1 month before exam start solving previous year question papers . And if you have passion you will definately clear the exam in 1 attempt .