I have 3 teeth missing that are missing in the back of my mouth, what is the best option to fill them?

Best option.: The best option for replacing missing back 3 teeth are dental implants. They are the most expensive but the best, last the longest. With implants you do not remove the restoration, it is fixed. The next option is removable partial denture. Most people do not like the fact that it is removable but it can work well for a while. R.P.D. Requires more maintenance and replacement after 6/7 years.

Where are they?: You don really say where they are missing. All together on one side, scattered here ad there...The best (and most$$$) is implants. Otherwise, fixed bridgework would next on the list. And finally, removable partial dentures, are in there also. They all work and if you get used to them then they are all available.

Implants: If the bone is sufficient, the absolute best option is implant placement and restoration.

Depends: Depends on their location and the health of adjacent teeth. Can't tell you what would be best without an examination, x-rays, gum/bone assessment, etc. In other words, see your dentist to discuss options. In most instances, implants/crowns are the best option.