a 3000 watt converter can pull up to 250 watts at 12V so add 10% minimum to that and you get 275amp fuse so what ever the next size up from that is. as for wire, you have to determin how far the batteries and inverter are apart and take the total run leng into account so if you have them 3 feet apart you will have a run length of 6 feet. then look up how big of a wire size will handle 250 amps for distance of 6 feet and then you have to take into account the voltage drop your happy with dealing with. for example if as above the return distance is 6 feet (or less) and you want to have a 1% voltage loss then you need a 4/0 cable.
