I just recently graduated after only 3 years of undergrad and will be attending either WashU or UCLA (waitlisted) next year. At my UCLA interview, my interviewer actually asked me where my 3rd year grades were and I had to explain that I was graduating after only 3. I would definitely say that getting in after 3 is doable, but it IS going to be an uphill battle.
My best advice is to apply, and do a lot of activities in the meantime. If you don't get in, opt to take some more classes or take a year off and pursue some cool and interesting activities and enjoy the free time. Use the time to improve your application and skills. Graduating early is a win-win situation, because even if you don't get in, you will have a free year to relax, regroup, and pursue whatever YOU want to pursue.