Graduating a single semester early will not make any appreciable difference. You will be applying right along with same group that you would if you were to take the entire 4 years, and will have completed 3 years in college at the time you submit your application.
Graduating in 3 years or less can make getting an acceptance more difficult, at certain schools in particular. It doesn't automatically make a person look brighter and more gifted because almost anyone can complete a degree in that amount of time if they schedule right and focus on taking only the classes necessary to complete their degree.
The least important reason is that there will always be some people who correlate age with maturity, even if it is subconscious. The other reasons are that some will question why a student felt the need to push through in less time. Be prepared to present good reasons that don't come across as having been pressured by family, being a one-sided academic nerd with a superiority complex, or being in a hurry to rush through your education to be a doctor. Those aren't selling points.
The overall greatest reason that it can get in the way is that applicant has one full year less experience in extra-curricular activities than most of the applicant pack, if they are applying after their second year in college. Their application would most likely have been stronger if they were to spend another year of being involved with the community.
In your case, I don't see any disadvantage. You'll be using that extra semester to strengthen your preparedness. That extra experience will offer you the opportunity to send updates to schools if you don't already have an acceptance in hand by the time you start your new activities.
The question has already been answered, but I'll add my voice to saying that the majority of applicants, and the majority of accepted students,
are still coming straight out of college.
