Sadly, at the time yo are limited to 3 PCs per subscription and only 1 subscription per LiveID. So, in your case, that would be 9 LiveIDs to manage for 9 subscriptions (with 2 spare licenses).
I have no details, but with the release of the next version of Small Business Server in the 2nd half of 2008, OneCare comes as part of the licensing for at least one of the versions. I assume that the activation and licensing of OneCare in that model allows for multiple copies of OneCare on PCs that are licensed to communicate with SBS. Since that product is still in private beta at this time, that doesn't help you now. :-(
-steve