The Samsung Galaxy S3 has been out of race for updates since a long time now. But not if you have rooted your device. Recently we saw a AOSP ROM based on the Android 5.0 Lollipop. The ROM is in it’s early stages and might a bit unstable.
It is certain that the S3 won’t be receiving the Lollipop Update officially from Samsung, but you could always try your luck by flashing a ROM and upgrading it to Lollipop. Developer arter97 has developed the ROM specifically for the S3. It is based on the original AOSP Source Code of Android 5.0 Lollipop, so it won’t be having the Touchwiz user interface in it.
As it is, the ROM is early stages many things have not been working, like the Camera, external sensors, WiFi, and also the Screen Off Functionality. But the bugs would removed soon, as it is still under development. You could have detailed look and keep the track of the ROM development from the source link below.