Some people over at Gamefaqs posted a link to a torrent of a guitar hero 3 demo, saying that you simply need to download it, burn it and pop it into your 360, and go into the demo menu to start it. I can't confirm this, but I figured you guys might like to know. I'd recommend disabling Live while playing it just incase
This is where the topic originated. Two download links can be found there. Just figured you guys would be interested.
Hope this is somewhat helpful to you guys.