Though Samsung is no more supporting it's S3, there are many developers who are.One such developer is Temasek.
There are many Lollipop ROM's available for Samsung Galaxy S3, but this the most stable ROM, I have come across so far. It is based on CyanogenMod 12 and the current lollipop version is 5.0.2 (5.0.1 is coming soon).
Everything is working properly. Including the camera.
So just watch the video and follow the simple steps to Install Lollipop rom on samsung s3.
Before proceeding, Make Sure:
1. Your phone is rooted
2. you have custom recovery tool installed
3. Backup any/all important data on your Android
Watch this video to learn, how to backup all your data on your android device:
you can download the ROM and GApps from here:
(This is the XDA page of Temasek)