CAT 3 Single A-320
The reason for this is during the approach mode (only) the busses split into three and there is no closing of the bus ties. So in order to have Cat3 Dual you need two separate power supplies Gen1 and Gen2 powering each side to get Cat3 dual.The a/c maintains this condition until on the ground.
With the APU Gen whcih is alraedy running on a constant speed unit the power cannot be split to either bus separately as with for example a Boeing product.
Hope this is helpfull.