Why won't phone charge

Its very common for an S3 to have a charge port go out which is were you connect the charger. I work at a repair shop and we get them in all the time with that issue. Its not an easy repair mainly cause a new one has to be soldered on which isn't always successful due to the fact the leads may be to damaged. a good alternative is to buy an external battery charger and an extra battery. Then just charge the battery outside of the phone. It will most likely be the cheapest bet unless you are experienced with soldering.