My daughter had a friend who graduated in 3 years, including a semester abroad. My son (graduating next month with Economics and a minor in Public Policy) could have easily graduated in 3 years, but is having such a great time wanted to stay for the fourth year.
You have various requirements. Your major requirements, general ed requirements, etc… You can talk to someone from your major at Cal Day or Orientation and they can tell you, for your specific situation, whether it is a viable option.