The Samsung Galaxy S3 gets Android 6.0 Marshmallow via CyanogenMod 13 nightly build
Granted, Samsung's most popular smartphone in history did not receive an official Android 6.0 Marshmallow release, which would be impossible given that Samsung stopped supporting the Galaxy S3 prior to Android 5.0 Lollipop. Instead, the handset has received its share of Marshmallow through CyanogenMod 13. The latest, Android 6.0-based version of the custom ROM was recently updated to support the Galaxy S3.
The Samsung Galaxy S3 created quite a stir in the market when it initially launched back in 2012. At the time, it had the specs to take on any smartphone in the market, including here a 4.8-inch display with a resolution of 720 by 1280 pixels, a 1.4GHz quad-core CPU paired with 1GB of RAM, an 8MP primary camera, and a 2100mAh battery. Samsung sold 50 million Galaxy S3 units in the first year of commercial availability, and the handset is one of the best-selling phones of all time.
Compared to modern flagships, the S3 is a bit of a dated device, but since a lot of people across the globe are still using the Galaxy S3, CyanogenMod's gesture isn't likely to go unnoticed, especially given that Android 5.0-based CyanogenMod 12 only came with limited support for the Galaxy S3.
Before heading on to the source link below, please bear in mind that this is a nightly CyanogenMod build. It's not stable enough to be recommended for daily drivers, but feel free to experiment with Android 6.0 Marshmallow on your backup Galaxy S3.
source: CyanogenMod 13