lol, ill say the same thing everyone else has said here.. maybe just make it a bit more clear
all the halo 3 saved movies are is data.. coding that tells the theater program what is where at any specific time in a game.. it remembers this, and thats what allows you to fly through the game in theater mode, as opposed to watching a flat video from nothing but the cameras point of view..
if saving a video and uploading it to your file share was a bit more like skate, where you record and edit your video.. you could do it, but instead they made it so you download the ENTIRE game data itself, so you fly around and see what you want from your point of view..
both ways are cool, but for what you want to do by far the easiest way would be to get a capture card.. i say the easiest way because im sure there are other ways to do it, but they will involve developing a program that can turn that coding into a video.. or something like that, i dunno >_>