• Hello tritika good to see you ask question like this.I think you want to build yoir career well and it is good to you and your future.
• According to new rules you can only appear in jee mains for three consecutive years only.
• If you fail to attempt in between there is no other way to continue JEE mains
• According to you,you have started writing mains from 2017 and written it in 2018 again then you are only eligible to write mains only once.
• That once is none other than 2019 but not in 2020 so you cannot attempt JEE mains again.
• Sorry to say this but it is not possible.
• But instead of mains you can attempt other exams do not get regraded you can have other chance in the life.
• Hope you got the answer you are searching for.                  
• Thankyou

No dear as per the jee norms you can give jee mains three times in continue manner suppose you done 12th in 2016 then you can give jee in 2016,2017,2018, not beyond 2018. If you would do 12th again in 2020 you can't give jee again because you break the continuity and you don't have any further chance to give jee. For more clarification visit official site of jee.

All the best