Dear Aspirant

First of all, you mistakenly wrote chemistry twice skipping physics subject.

Considering you are good at 2 subjects but are weak in the 3rd. Actually a balance of all these 3 subjects lifts you to get an outstanding score in NEET.

If you think you are good at 2, you should start preparing yourself for the 3rd one aswell. Joing any institue of coaching is not necessary but yes it can clear alot of your doubts and gives you more clarity about the subjects. 

You can refer online videos for help or seek somebody good in the respective subject.

All the Best, further queries are always welcomed.