My S3 is also about the same age and is running mostly fine, except for two things: 1- the USB port is damaged and the phone sometimes doesn't charge or charges very slowly; I know it's probably a soldering problem, but I can't be bothered to fix it, and 2- the Facebook app frequently crashes and reboots the phone. I suspect this has more to do with Facebook being awful on Android, since I've seen it happening in other phones as well.

Point is, I won't change my S3 until it crumbles to dust, if I can help it. I don't understand the obsession with buying the latest mobile every 1-2 years.