Your Xbox 360 is probably lagging. make sure that you have broadband (Such as DSL) if you do try connecting your 360 straight into the modem (if it isn't already). if that doesn't work,try checking the Halo 3 disk for scratches. If there are, try cleaning the disk as best you can. If theres a huge deep scratch going in circles around the disk, (or going straight down) you have a bad Halo 3 disk and should see if Microsoft will replace it.
Also if you have a wireless connection in your house, the game will do that if there is someone on a computer on the Internet.