Dear Aspirant,

One can give JEE MAINS 3 years consecutively from the year he/she was in 12th class.

IF you are in 12th class of 2021, then you can give JEE Mains in the year 2021,2022,2023.

For JEE Advanced one can give the year he/she is in 12th class and next year.

If you are in 12th class of 2021, then you can give JEE Advaced in the year 2021,2022.

FInd more here :

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

JEE ADVANCED- https://engineering.careers360.com/articles/jee-advanced-eligibility-criteria

JEE MAINS  tips - https://engineering.careers360.com/articles/jee-main-preparation-tips