Turn Off Computer Once You Hit Part 3?
 

Hmm, I'm not sure, but I heard somewhere that once you hit part 3, you have an unusable computer. Does this mean you cant turn it off untill you finish?

Well the portion your building is unusable most likely but your still building from another distro install, so you can always reboot, turnoff and then boot to the distro your building from to continue, etc. Just don't try to boot into your unfinished LFS install though.. ;)