i scored 87 percent in cbse board and dropped one year ,plz tell me what min marks i should score so that i can get any of the top 3 nits pref. branches : cs , aerospace ,mechanical

For you to get into NIT(top three) in CS,Aerospace, Mechanical, then you have to score more than 220 above out of 360. This benchmark may vary but not that much.On a safe side you need to get 250-270 for getting a sure shot go for these branches.