As mentioned that 12th portion is completed then it will be easier to get further with portion of entrance exam. 

Now Boards Percentage is important too so focus on Boards along with entrance exam. 

Prepare a timetable and be disciplined to follow it. 

After revising every chapter solve Multiple Choice Questions based on that chapter. If any doubts arise then get them cleared at that point of time itself as postponing it will have difficulties in future. 

Prepare handwritten notes as these are very helpful. Prepare separate notes of formulas so that you can revise before the exam. 

Refer and study NCERT textbooks of Physics, Chemistry and Biology of 11th and 12th standards. Understand the concepts. There are many other good reference books too which can be referred. Students tend to refer almost 5 to 7 books for each subject which gives rise to confusion, I would suggest you to refer a good book and understand it thoroughly. 
Books which can be referred are as follows :-
For Physics :-

• Fundamentals of Physics by David Halliday, Resnick and Walker,  Objective Physics by D. C Pandey, Numerical Physics by I. E Irodov

For Chemistry :-

• Physical Chemistry :- Numerical Chemistry by R. C. Mukherjee.
• Inorganic Chemistry :- Inorganic chemistry by O. P Tandon, JD Lee.
• Organic chemistry :- Organic chemistry by O. P Tandon, organic chemistry by T W  Graham Solomons.

For Biology :-
Trueman's biology volume 1 and volume 2 , NCERT at fingertips.

Give equal attention to every subject as they are all equally important. 

Practice numericals in physics as many as you can. 

In organic chemistry there are many types of chemical reactions and each reaction has a specific mechanism. To get thorough with reactions write them everyday. 

Is three months time sufficient? Well it would depend upon various factors like how much portion is covered and related things. So give your best every single day. Don't take stress as it affects studies.
Practice past year papers of the entrance exam. Set up the clock and aim to finish it on time.
Practice diagrams in Biology from NCERT textbooks as they are important too. 

If you still have any queries feel free to ask in the comment section down below. 

Hi Hitesh,
                 Thats very good if you have completed your 12th syllabus and revised everything now for neet and aiims go for paper solving because these are the exams that require a lot of practice and conceptual study of the matter given to you. So all the very best !

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