The passengers re-taking the third plane was a stroke of luck. That flight was actually delayed long enough that passengers were making phone calls to friends and family and heard that the first plane (or two) had already crashed, so they knew what was happening. That's when they decided to storm the cockpit. Had they not been delayed, they probably wouldn't have known and not retaken the plane.

edit: but agreed - point being we know now if a plane is hijacked, there's very little chance of it coming down peacefully.