If a guy failed in 2017 & 2018 12th cbse,can he appear in 2019 board exam?And what if he got more than 75 this time and clear Mains?

You can't appear from the same board, i.e. CBSE.

But on the other hand you more options to reappear for the same.

Like NIOS, NWAC and any other state board.

yes, if student want to  appear for 12th cbse  again he can. and for JEE he should only satify  eligibility conditions(age,12th percents etc.)

AS JEE MAIN 2018 Application form is about to release on 1st December, 2017 and candidates will be able to register themselves for the exam till 1st Jan, 2018. S it becomes important for the student to know and understand the eligibility criteria for the JEE MAIN exam.

The candidates are majorly concerned with Age Limit, Education Qualification, Number of Attempts and one must ensure complete eligibility before appkying for the exam as there are only 3 attempts for the JEE Main exam. The eligibility criteria as per CBSE is as follows-

Age Limit- The age limit to appear in JEE Main 2018-

The candidates should have been born on or after October 1, 1993.

There is age relaxation for SC, ST, PWD candidates of 5 years is applicable.

2. Year of Passing the Qualifying Examination –

The candidates should have appeared for Qualifying Examination in 2016 or 2017 or appearing for the final exams in 2018, but they need to submit the certificates before the due date.

If a candidate did not pass the Qualifying Examination in the year 2015 but passed in 2016 is not eligible.