IB,
It should be possible if the computer you are trying to back up to is shared. You should simply try browsing to that computer you are trying to use as a backup.
When I had just thrown out the suggestion that one location could be on the same computer, I was simply saying it could make things easier. I don't necessarily think you have to do three backups and one to the same hard drive would be the least of the ones I would do.
If your only backup is to the same computer, you are definitely in for trouble. The whole idea of a backup is to cover deleted files, corrupted files hard drive crashes. So, if you crash a hard drive (which is not uncommon) you are out of luck. Even if you have a second drive, it is not a good idea to rely on one computer.
I would highly recommend that you back up to something external and not another computer as people are more likely to use that computer and delete your backups. I would recommend picking up at least two external USB drives. You can get these very cheap and they are super large now. You don't want to rely on just one backup.
