Nisha,
You have a valid query when you say that B schools might ask for 12+4 education. I am very sure that some schools specifically ask for 12+4 from indians and that would be 12th and then engineering or any other professional degree which takes 4 or more number of years or Msc/MA/Mcom.
As far as 12+3 is concerned some schools stress that candidate should have completed another year of Msc/Mcom/MA (in some cases they want the degree to be completed, and not just one year to satisfy the 12+4 requirement). Now some schools would think that ur 12+3 is equivalent to US bachelors and would give you admissions on that basis. (Kellogs is one such example, but again plz confirm it from there website, I might be wrong)
So, the gist is that your qulaifications have to match US bachelors degree, and it depends on US school's guidelines as to what education level from india they think to be equivalent. I believe every leading business schools in US has a policy on this and you might want to check there websites, or email admissions committe on any doubts.
