JEE MAIN ask question from all the topics you cannot expect a good score just by selective study.

you should focus on each and every topic because they ask question from even chapter such as chemistry in everyday life.

There are few topics which are very important for JEE MAIN so you need to focus on such topics ,this topics includes electrostatics, rotational motion , optics modern physics.

For Chemistry you need to focus on chemical bonding Thermodynamics , redox reaction,  Ketone carboxylic acid.

For mathematics focus on Calculus and co-ordinate geometry.

As time is very less concentrate on the important ones and also the chapters with more weightage 

Practice more previous question papers and attempt mock tests to be prepared for the exam

Also, make a daily schedule and give equal importance to all three subjects. For more guidance please go through the following link.

 https://engineering.careers360.com/articles/how-prepare-for-jee-main-in-3-months