Hey there

3 months preparation for JEE mains can be harsh and in reality very few student can crack the exam just by studying 3 months. So Start preparing for now you have time just put your 100% everyday things will be easier each day if you'll put your efforts regularly if you'll leave things for tomorrow they will become burden. For boards I'll advise that you can get 75% or more just by studying last 3 months if you have build even a little concept earlier but JEE is tough do not take risk

Good luck