It's most likely the cable connecting the Xbox to the TV. If you have a friend with a 360 you could try their cable and if it works you've found the problem.

If it doesn't work then it's either the TV or the Xbox. To rule out the TV, try connecting your Comcast box to input 2 (assuming input 2 has the same sockets as input 3). If that works then it's your Xbox, so send it away for repair. If not it's probably the TV, in which case you could connect your Xbox to input 1 or 3 and you'd be able to play.

You should also rule out the facepalm things like checking your 360 cable is plugged into the TV in the right order, i.e. making sure the video plug isn't in one of the audio sockets, etc. If you're using standard A/V (red, white, yellow plugs) then red and white are audio; yellow is video.