I don't know the right version of sentence

1. In my opinion you should think of things which you are interested in.
2. In my opinion you should think which things are you interested in.
3. In my opinion you should think about what is really interesting for you.

Sorry, I would say none of them are correct. I think you want to say:

'In my opinion you should think about what really interests you.' When you say 'think about' here, it might be better to say 'consider'.