⟩ Why should you not be over ambitious while answering dream job question?
While it is good to show confidence and a desire to progress in your career, going into a job interview and proclaiming that you want to run the company within three years probably will not go down too well.
Pick a role with responsibilities that would be a natural progression from the role you're applying for. That way, it puts the interviewer's mind at rest that you won't be unfulfilled in the current role.