Samsung Galaxy S3 has a 4.8″ 720p display. It was the first device to have a quad-core Exynos 4412 processor, with each core clocked at a maximum 1.4 GHz. It packs NFC, wireless charging capability, and the latest wireless radios. It ships with Samsung’s Nature UX interface. At the time of its launch, it ran on Android 4.0.4 ICS.
Samsung Galaxy S3 did not receive KitKat official update so it is impossible that Samsung will provide Marshmallow support. However S3 users need not worry as CyanogenMod team have created custom ROM which will update S3 to Android 6.0. Detailed tutorial is given below.