Dear student, if you have completed your exams in 2017-18 session, then it means that you have given your examination in 2018 and your passing year of class 12th board exams is 2018. In jee mains you are only allowed upto maximum three attempts to give the jee mains exam. The three attempts of yours are counted from the year in which you have passed the class 12. I'm telling you this because I had given my class 12th exams in 2018 and was part of session 2017 - 18.
If you think that you can apply in 2021 then you are completely wrong. You are not eligible to give the jee mains exam. Your all the three attempts are over whether you have given the exam or not. Your first attempt was over in 2018, second attempt in 2019, and third attempt in 2020, so you are not eligible to give jee mains 2021. It doesn't matter that you have given the exam in 2018, 2019 and 2020 or not.

Check the eligibility criteria for appearing in jee mains 2020 from the following link so as to get a rough idea for eligibility criteria for jee mains 2021:-

https://engineering.careers360.com/articles/jee-main-eligibility-criteria/amp

Yes,you can give the exam but it is not advisory.

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