Apparently you did get the credit in rolling credits section, as agreed by you in the said contract? So the makers did satisfy the contract, and you were paid aptly for the credits as shown in this video and this agreement??
The one gotcha here is that you cannot make them liable on moral grounds for not putting the credits in the beginning of the movie, which technically were 'not' rolling credits. It would have brought the makers so much more goodwill and credit if they had acknowledged the 'Based on...' credit in the beginning credits, but that was NOT required by the contract, so oh well!
Anyways, this was unfortunate. And whether or not you read the contract clearly before putting the pen down, perhaps you should be more careful in the future. Do -not- assume anything. As far as credits for the original story go, you will be recognized as the first author, now, and ever. However, if the film-crew who adapted the book to a slightly different screenplay are awarded, they deserve the awards, that is exactly what the contract was about, wasn't it: to give away rights to 'adapt' the book to a movie?
You are an accomplished writer, it may be best to put this issue to bed now.
Disclaimer: I am not related to either Chetan Bhagat or the Crew of '3 Idiots'. All views expressed above are solely my own, with no direct influence from either parties.