When you bought X3 you got a licence to Melodyne Essential and had a serial number allocated.
As your version of Platinum is an upgrade you don't get an additional licence of Melodyne. So you only ever did have one licence and one serial number.
Of course that's the case with 3rd party programs and plugins that are included with various versions of Sonar. If you had purchased a full version (non upgrade) of Sonar each time you would have received a licence each time for Melodyne.
I've just included this for information of those who may come after and read this thread.