Unfortunately, there is no elegant solution.
A OneCare subscription is good for up to 3 PCs and becomes a "Circle" if you use the multi-PC features in OneCare.
You can only have one subscription for OneCare associated with a LiveID. You cannot, at this time, add licenses to a subscription.
So, you would need to activate/subscribe with a different LiveID for each 3 computers. This precludes the ability to use a single PC as a hub to manage all of the PCs as each subscription becomes its own Circle.
-steve