Depends on how your monitors are configured. Assuming you are using a 1920x1080 res movie, on 3 1920x1080 landscape mode Eyefinity will give you a 5760x1080 screen. When you go to full screen the movie it will fill middle monitor (both side screens will be black, assuming you set the video player to maintain aspect ratio of the movie).
Or you could use 3 1920x1080 screens in portrait giving you a 3240x1920 screen. The movie should stretch across all 3 screens. But you will have two gaps running through your image.