Why does my straight talk samsung galaxy s3 get so hot?

Your ram is being used up by apps constantly running in the back ground. Try get rid of apps you don't really need and also download these two apps from google play, "Cleanmaster" and "Batterydoctor" they are the two best apps for keeping your device from running to hot and filling up with useless files etc etc. This should help you :)

Lots of phones get hot, it all depends on what apps you have running and talk time. When you run wifi and bluetooth it uses up power an many phones run hot. In my opinion to hot. My Fire Phone runs hot and the battery lasts shorter than my older phone without all this extra apps. If you turn off bluetooth it will be less hot and if you dont need wifi all the time turn it off until you do need it. No way around this one.