How do I deactivate Creative Suite 3 from a computer I no longer have access to?

As far as my experience in this forum goes, Adobe support has the ability to deactivate inaccessible installations.  Unless that has changed just recently it should still stand.  You can contact them thru chat and get it taken care of.

Serial number and activation chat support (non-CC)

http://helpx.adobe.com/x-productkb/global/service1.html ( http://adobe.ly/1aYjbSC )

Hi paddylock

Although it's not possible to deactivate CS3 in this situation, the activation count will allow you to install it on a new computer.

Thanks

Bev