You know, I agree that that is a silly dialog.
Here is how it works.
You install OneCare on PC1 and activate your subscription with your LiveID.
You make the PC a hub in your Circle.
You install OneCare on PC2 and activate using the same LiveID. Repeat for PC3.
In a short time, when you open OneCare on the hub, PC1, you can click on Manage Your OneCare Circle and all 3 computers are listed. Basically, any computer that is activated with the same LiveID belongs to your subscription and will appear on any PC in your subscription that has been made into a Hub.
Note that all PCs in the Circle need to be at v2 and none can be running the Beta of v2 in order for this to work.
-steve