It seems like you're dreaming of being a doctor ASAP, but since you're still young, there are some things to consider. If you do want to graduate high school in 3 years, great. But I would recommend taking that extra year and travel or do something that you love. It might seem like a big deal now, "wasting" a year, but I guarantee it won't matter if you become a doctor at age 25 or 26. What will be memorable is if you taking that time to explore the world or go and do some shadowing or volunteering to see if this is really something you can commit to for the rest of your life. I don't know too much about the combined BS/MD or BS/DO schools, but I do know that when you apply for med school (after graduating college) they want to see that you have been exposed to the medical field. You can't just get into med school with a good gpa and MCAT score, you need a lot of extracurricular activities. They want to see that you have other life experiences, hobbies, etc.
On a side note, I took some time off after I graduated college and traveled for almost a year (backpacking through some diff countries) before I started medical school. It was the best experience of my life and I would NEVER regret. I also took some of that time and did medical internships overseas, and I always got asked about that during my interviews! Med schools want to see that while you may love medicine, you also have a life outside of it and things you like to do so you don't burn out.
Also, don't go into medical schools having regrets! "I wish I had done that before med school"
Because after you start, you won't have time! It's a lot easier to take the time off now to travel or whatever you want to do after high school or college than after med school.
Don't feel so rushed!