Sent from my Galaxy Nexus using XDA Free mobile app
Well dude I think your assumption is right.
It might be the charging circuit only which may be dead.
Because as you told that you have tried diffrent cabels and chargers but still your charging is not working which means that the usb micro port through which we charge and connect our phone to computer is damaged and should be replaced .
There are many possible reason of damaging usb port like you may have drop your phone or by using faulty cabels and chargers or if u have rooted your phone then there is possibility of changing the voltage of current to more than 1000 Mhz etc etc.
You should try to flashed a complete stock rom by downloading from sammobile or directly using kies to update or re-flash your phone
you can try that may it may help you.
You can try micro usb port which is available on ebay but it would be extremely very dangerous until and unless you know what you are doing.
My suggestion is to give your phone to the samsung care and tell then to repair it.
If your phone is in warranty then they will do it for free. You just need to provide them a copy of bill at the time you purchased s3.
Press THANKS if I helped you