In order to crack JEE Main you should be determined and self motivated. 

If you want to crack JEE Main in 3 months you need to work hard for that. To crack JEE you have to practice as much as you can. This will help in strengthening all important concepts. Focus on important topics. Refer previous year papers and MCQs for your practice. You must join All India Test Series for better time management and accuracy.

Below are the important topics you can refer

Mathematics: Quadratic Equations & Expressions, Complex Numbers, Probability, Vectors, Matrices in Algebra; Circle, Parabola, Hyperbola in Coordinate Geometry; Functions, Limits, Continuity and Differentiability, Application of Derivatives, Definite Integral in Calculus.

Physics: Mechanics, Fluids, Heat & Thermodynamics, Waves & Sound, Capacitors & Electrostatics, Magnetics, Electromagnetic Induction, Optics and Modern Physics.

Chemistry: Coordination Chemistry & Chemical Bonding in Inorganic Chemistry, Electrochemistry, Chemical & Ionic Equilibrium, Mole Concept in Physical Chemistry and Organic Chemistry.

You can also knw the preparaton tips from the given below link

JEE Main Preparation Tips

Good Luck!!