What happens when you try to install to the 3rd PC? You should not have had to purchase a new copy. However, now that you did, you can obtain a refund for a retail purchase if you are in North America per the information here -
http://www.microsoft.com/mscorp/productrefund/refund.asp
If you are not in North America, I do not know the procedure for a refund, you should use the Contact link on Microsoft.com for your country to find out the procedure.
Alternatively, you can use the retail key to extend your current 1 year subscription by signing in at http://billing.microsoft.com with your Subscription LiveID and clicking renew for the OneCare subscription and selecting the option to renew with a PIN or token, which is the Retail Key from inside the box.
-steve
