You have a few things you can try.
Different USB cable; it goes between phone & charger block/computer USB port.
If your charger plugs into a wall outlet, try a different charger, try another wall outlet.
See if you charger equipment works on another phone.
You may have a damaged usb connector on the G3.
If you conclude that, then you may have to take the battery out, and use a stand alone battery charger. When the battery is charged, you would put it back in the G3.