The OneCare activation dialog has a particularly stupid issue in that it does not accept valid passwords that are longer than 8 characters. Change your password at http://account.live.com to be shorter than 8 characters. Activate the subscription and activate the other 2 computers to associate them to the subscription (same LiveID for all 3 computers) and then feel free to change your password back.
Let us know if that resolves it.
-steve