The college and the hostel scenes were filmed at Indian Institute of Management, Bangalore, although the fictional "Imperical College of Engineering" depicted in the movie is situated in the city of Delhi. The ragging scenes were shot in the girl's dormitory and the resident girls saw the whole scene (not shown). The actors admitted to feeling uncomfortable while getting undressed to their underwear in front of them and getting their buttocks stamped.
Aamir Khan's character Rancho says in the movie that people study engineering, then management, and then become bankers, so what is the whole point of studying engineering. This is exactly what Chetan Bhagat (the author of 'Five Point Someone', the book from which the movie has been adapted) did. He studied engineering from IIT, management from IIM, and became a banker.
While shooting the drunk scene, Aamir Khan suggested that they get really drunk and that happened on the shoot which resulted in numerous retakes and the stock of camera roll got over and the unit had to go to another shoot in Bangalore (where the film was shot) to get roll.
Known for preparing for every role that he signs, actor Aamir Khan was avoiding eating his own birthday cake. When asked why, he said "I am surviving on milk and few bananas as I have been asked to lose weight for the role".
Before Hirani made his first movie (Munna Bhai MBBS, released 2003) he had written a draft about three college students studying in an engineering college. The movie was to star Hrithik Roshan, Saif Ali Khan, and Abhishiek Bachan. Filming was to take place after Munna Bhai MBBS released, however due to creative issues, between Hirani, and the producer the film was never made. Five years later Hirani announced 3 Idiots, based on three college students studying engineering.
Producer Vidhu Vinod Chopra has claimed that 3 Idiots will be legally released on YouTube within 12 weeks of releasing in theaters - a first in the history of Indian cinema but in reality was never done.
Sanjay Dutt was supposed to make a cameo in the film, but this never happened. Also, Arshad Warsi was offered both the roles of Farhan and Raju but declined due to lack of dates. Both have previously worked with Rajkumar Hirani and Vidhu Vinod Chopra in the Munnabhai film series.