Generally graduating early is not a plus for competitive colleges. The fact is that most people can graduate HS a year early with pretty minimal effort so it is not a particularly impressive feat.
By graduating early you will miss out on taking a group of the harder (AP or equivalent) courses in your HS that candidates to competitive colleges typically have AND you will have a year less of involvement/accomplishments in your EC activities (plus many people take on leadership roles during senior year).
Additionally the above comment is correct. Many top colleges look for applicants to have four years of all core subjects which you may not have if you graduate early. You should check the common data set (section C) for every school you plan to apply to.
If you are looking to go to a less competitive college then graduating early should not hurt you.