3 Idiots is very loosely inspired by Chetan Bhagat novel

Even though Raju Hirani's 3 Idiots is based on Chetan Bhagat's Five Point Someone, the film has turned out to be very different from the book

Even though Raju Hirani's 3 Idiots is based on Chetan Bhagat's Five Point Someone, the film has turned out to be very different from the book. On the big screen, the film has merely been relegated to being a source of inspiration for an original plot.

Although Vidhu Vinod Chopra bought the rights to the book, Raju Hirani wrote quite a different script to his film.
A source says, "If you walk in expecting the film to be just like the book then you'll be disappointed. The basic premise is maintained, but the characters, situations and the end is quite different. In fact, there is just a five per cent similarity between the book and what you see on screen."

Aamir Khan and Kareena Kapoor on the sets of 3 Idiots

In fact, the campus and the classroom setting for 3 Idiots is the only thing that remains true to the book;
everything else is different. The source adds that the film will say it's inspired from the book and not based on it.