I've never tried three O/S's, but it can be done. You should always install the oldest O/S first, so start with XP, then install Vista, then 7. Not really sure why you want Vista and 7, there's not that much difference between them. I'd probably just go for XP and 7, but it's up to you. Also, keep in mind that some Windows 7 versions also have XP Mode if you have the appropriate hardware (If you have a particular program that won't run on Vista/7).