I graduated in 3 years (just graduated this year) and it was the best decision I could have made. I went in with a lot of AP credit too, so I ended up not having to take many electives at all. I never took more than 15 credits a semester (leaving time for a job, shadowing, a sorority, etc.) and most of my semesters were just science classes. So I'm not sure what the person above was talking about that someone graduating early wouldn't have the chance to take hard science classes. I took two semesters of physiology, microbiology, biochemistry, immunology, endocrinology, neurophysiology, etc. I also graduated with a 3.9 so I'm sure that graduating early had no negative effect on my GPA.
When I was a freshman and figured out that I would be able to graduate early, I was on the same thought process as you and thought I would go straight into dental school after undergrad. But many people talked me out of it, and I'm so glad they did. I'm applying with the "normal" cycle for my grade, which is allowing me to have a year off. I was so burned out by the end of this year, and I don't know what I would do if I didn't have this year to take a break. I've been able to get a full time job at a dental practice this year and I also have time to do lots of fun things. Also, it's been really great not having any stress about missing classes to interview this fall. I have been able to really focus on my application/interviews without having to balance school during this time, and that is great!
Anyways, I just wanted to share my experience with you. Your situation might be different, but I want to encourage you to take the year off if you are able to graduate in three years.