If the trial version is installed and not activated on your other computers, then activating on the first PC will activate your subscription. Using the *same* LiveID to subscribe/activate the other two installs will associate them with your subscription.
If you have already subscribed to OneCare and it is running on the other 2 PCs, you activate on the 3rd PC with the same LiveID.
There is no general problem with expiration dates.
You can always check the status of a subscription at http://billing.microsoft.com after signing in with the subscription LiveID found under Help/About... in OneCare.
You can purchase for the full retail price by providing credit card information during activation on the first PC or obtain a retail copy of OneCare, often at a discount, to use the Retail PIN/Key to activate the subscription. Subsequent activations are done with the LiveID, not the key or purchase.
-steve
