In a OneCare Circle, any or all of the PCs can be made into a "Hub" PC to see the status of your other subscribed PCs. From any Hub, you can remove any other PC from your subscription. This will cause OneCare to become expired on that other PC. As long as OneCare is not installed and subscribed with the same LiveID (the Subscription ID) on more than 3 computers, you are within the terms of the license.
To make a PC a hub, open OneCare and click Manage Your OneCare Circle and then Make this PC a hub.
To remove another PC, open OneCare on a hub and click Your OneCare Circle, expand the details for that PC, then click the link to remove that PC from the subscription.
-steve