Please verify that it is the same LiveID by going to Help/About in OneCare on the other PCs or by signing in at http://billing.microsoft.com
Even if you had 3 PCs activated already with that LiveID, you would not get an error telling you to get a subscription - it would activate. The primary reason for encountering the "need subscription" page is that you are using the incorrect LiveID.
After confirming the LiveID, if you still can't activate, please contact support.
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve