Is the 3rd PC still running the beta code? If it is, you will need to uninstall it and reinstall from
http://onecare.live.com and then activate with your subscription LiveID. If not, and you are signing in with the correct LiveID for your subscription, please contact support -
How to reach support - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
If it fails to validate your subscription, select the option that you are using a trial or beta copy and you can proceed to email support without validation once you've signed in.
-steve