I think I misunderstood you.
you are talking about fullscreen games and I was thinking maximised windows. I didn't have xinerama in my use flags and it would split the window across all 3 monitors. sadly I was never able to get fullscreen games to work properly with xinerama either.
for a while I kept 2 xorg.conf files one for xinerama and the other just 1 screen so I could play games. now I'm down to just the 1 monitor and my tv with no xinerama. I send movies and games to the tv by setting DISPLAY=:0.1 ,but yes I lost the ability to drag from screen to screen. I apologize for the misunderstanding.