Open OneCare on the hub and click on Configure backup.
On the left, you will see:
Backup plan for all PCs
Computer 1
Computer 2
Computer 3
(the names of computer 1, 2, and 3 will appear)
The hub should be the first PC listed.
Click on the "change settings" button in the lower left.
Click on the Hub PC name.
Click on the Central backup device (recommended) selection for Where.
Click Next, and save the settings.
You may first need to uncheck the "turn on backup on this PC" selection, click Next to save the settings and then return to that page to select the Central Backup location for the hub PC.
-steve