I know some phones will power on with a wall charger even after completely removing the battery. Not all phones will do this though, and I am not sure about the LG's. This could indicate that you have a faulty battery that needs replaced. If the phone's power button was the issue it would likely not power on at all, but may indicate its charging when connected to a charger. I would try replacing the battery or if you have access to another LG G3 you could test your battery in the other G3 phone or use the other phones battery to try and power on yours. If you have a multimeter you could remove the battery and check to see what the batteries voltage reads before purchasing a new battery. A phone will usually not power on if the battery's voltage is anywhere below 3.4v. If you have been using a third party charger to charge the battery it may have been providing the battery with the incorrect charge current causing damage to the battery. I believe that a new battery should fix your issue, and its a good sign the phone can still power up when connected to a charger.