This can be done from your TS3 server control panel.
1. Sign into your WHMCS billing account here.
2. Under Account Overview click View to the right of 'Number of products/Services'.
3. Click View Details to the right of the Status of the server.
4. Click the button labeled Login to Ventrilo Control Panel (this is your TS3 control panel)
5. Click Manage Server -> Privilege Key
6. Choose what kind of key you would like to generate from the drop-down menu, and then select Genetate Key.
7. Copy and save that key in a safe place.