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If you are able to satisfy below Criteria, you can give JEE Mains 2019.

As per the JEE Main 2019 Eligibility Criteria,

Candidates must have passed class XII or qualifying exam in 2017 or 2018.

Candidates who are born on or after October 1, 1994 are eligible to apply for JEE Main 2019 exam.

All the best.

for Jee Mains at maximum of 3  consecutive attempts are allowed. 

Incase you have appeared for 12th in 2017, then you can appear for 2017, 2018 and 2019 years for jee mains. However if you didnt appear in 2018 then you cannot again appear for 2019 as only successive attempts are allowed. 

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