Do you know a method to play three (QuickTime) videos in sync on three different machines? At least I would trigger the space bar to start them in sync.
Did you try Multiscreener? I have used this in the past and worked flawlessly for 3 day runs. http://www.zachpoff.com/software/multiscreener/
I don't know how well synchronized it would be, but you could try using
at (or launchd or cron) to run an osascript command at a specified time.
sudo launchctl load -w /System/Library/LaunchDaemons/com.apple.atrun.plist at 16:12 <<< 'osascript -e "tell app \"QuickTime Player\" to play document 1"'