Hello,

I would suggest that you sit for all the three exams. This is because, you never know which exam would have questions that you are familiar with and you know the answers to unless you attempt them. Doesn't matter if you have not prepared that well for JEE Mains, but still give the exam. What if by some luck the many questions are  asked from the topics you have prepared and you qualify it with a decent score, then You will have more options with JEE Mains in the JoSSA councelling.

SO appear for all the three and give it your best.

You still have time. Practice as many previous years papers as possible.

Good Luck