Are the 2 PCs that are currently running OneCare and in a Circle beta installs from prior to the middle of November? If so, you will not be able to add the 3rd PC, installed from http://onecare.live.com, to the same Circle as they are connecting to different environments on the back end.
Sign in with the subscription LiveID shown on the two original PCs at http://billing.microsoft.com - if you show a beta subscription and also a regular subscription, you will need to uninstall OneCare on the two other PCs and reinstall from the production site.
-steve