From the main menu, select "Episodes." Scroll down to the third or fourth entry on that disc's list to find Episode 18.
Some Season 3 discs feature commentary from the executive producers, Kevin S. Bright, Marta Kauffman, and David Crane, giving you behind-the-scenes trivia about the hypnosis tape plot. Watch Friends S03E18 DVD
This episode is typically found on Disc 4 of the Friends Season 3 DVD box set. From the main menu, select "Episodes