The first thing to look for would be to launch eLicenser Control Center and check that your license is displayed there.
The second thing to check is that the latest version of the eLicenser Control software is installed (if you simply ran the Cubase SX 3 installer, it would have installed a very old version of the eLicenser Control software that doesn’t understand yet what a “Cubase 8.5” license is supposed to be).