Before you rejoice, let me say that the update doesn’t come from Samsung but from the developer community. The galaxy s3 might be an ancient smartphone in the fast evolving tech world but is very capable to run the lollipop android.
Samsung doesn’t support its phones when they are out of the general 2-year life cycle and permanently leaves the phone on the ancient version of android. But during such difficult time, the developer community becomes a boon. Then have released a stable version of the android lollipop for the galaxy s3.
As for the device, the galaxy s3 was released in the year 2012 running android 4.0.4 ice cream sandwich. Samsung has officially updated the phone to android 4.3 before abandoning. now a new firmware called as the new dirty unicorn firmware has updated the phone to lollipop. All the new features such as the material design and app menu are available in this update.
Actually, I would recommend anyone with the galaxy s3 to install this custom rom. Samsung has loaded the phone with lots of bloatware that eats up the precious processing power slowing down the phone considerably. With the custom rom, user can have the nexus like experience with absolutely no bloatware.
You get to experience clean android and I prophesize that the marshmallow update would also be available soon. In cane you are wondering, the galaxy nexus phone has recently received a custom rom that can install android marshmallow onto the device. be sure to back up your device and do read the instruction carefully before attempting anything. Many phone have been bricked more often than not.
Follow the link for instruction and the file for custom rom.