This usually happens when you overload the processor work and also when the lights LG G3 without keeping the display on. The case is that when you want to use the phone is much warmer than it should (and the main reason is that the battery temperature increases).
Here the best solution is to reduce the brightness of the screen as it is one of the culprits that are used in excess processor and battery, resulting in many cases overheating. This can be done on the screen of the Settings. You certainly would not hurt to occasionally revisases the Battery section to see if there are many applications that are using it. If so, close any of them to make everything work much more efficiently.