@melid-Vega I don't think Magix would sell you Sony Vegas Pro 12? When did you purchase it? The old SCS registration details were migrated over to Magix and if you go to your account there you may find your original serial number.
Just what @EricLNZ says. And if you're receiving P3 serial numbers you most likely have bought VEGAS Pro 14 or later from somewhere. Your software can be downloaded by entering your serial number(s) at the MAGIX Download Center