The Samsung Galaxy S3 came out in 2012 and after the release of KitKat 4.4.2; owners have been promised the update. This is only going to happen once the update has been rolled out onto the Samsung Galaxy S5.
When it is out, the Samsung Galaxy S3 will join other devices, such as the Galaxy S3 Mini, Galaxy S4 Zoom, Galaxy S4 Active, Galaxy Note 2 and Galaxy Mega, as they are all said to be getting the update.
Of course KitKat 4.4.2 is not just for smartphones, tablets belonging to Samsung will also be updated. The Samsung Galaxy Note 8 along with the Samsung Galaxy Tab 3.7.0 should also be updated.
The rollout of KitKat will not happen on all handsets at the same time. It will arrive first on the new handsets and then spread out to the older one. Thanks to this it could be some time before Samsung Galaxy S3 owners get to see the update on their handset.
