The reason is because s3 you have bought probably is not for your country but imported. The same for me . I have discovered that my s3 taken on Amazon is not italian version but poland. So It will never work in Samsung Pay. Poor me.
This is why I didn't bougth S3 Gear and decided to see some smart watch that accept Google Pay. It would be great that all vendors unlock this (one device allo i.e Apple Pay / Samsung Pay / Google Pay / Whatever Pay) and allow us to use what fits letter but sad seems to be the otherwise or just create Universal Pay and stop with this things pay.
Alot of banks won't touch Samsung Pay. I'm with Halifax and they said they never will. This is the reason I will definitely not buy a Samsung product that doesn't support Android. Spiteful Samsung!