No, there will be no issue as you are eligible to get seat in NITs as you can give Jee main thrice consecutively from the year when you are in 12th but if you talk about placements then there are few companies which may ask the reason of gap so you must be having a valid reason for the same.

Good Luck!!!

You can take three attempts to crack JEE Main you will be eligible for NITs but make sure you should have 75% marks in your boards exam and you should be in top 20 percentile. 

No, there will be no problem in placements. 

You can also check the eligibiity criteria of JEE from the given below link

JEE Main eligibiity Criteria

If you taking a drop make sure you are determined to get admission in top NITs. Work on your basics you can take help of NCERT books. Go through previous year papers to understand the pattern and you can also solve complete mock test series to know your strength and weakness.

Good Luck!!