Hey Uma.

See u can attempt JEE MAINS for three consecutive years but for the last year u will be eligible only for the mains rank card and not be able to apply for the JEE ADVANCED. U will face no problem while taking admission in the NITs under the third attempt but it might hamper ur placement scenario as the companies mostly recruit the fresher's and 1 yr droppers.

Hope u got my point. Ask if any query.

Good luck