I’m not sure if you’re referring to your old fans being plugged into a fan hub or onto your main board, however you should still be able to plug the 3 pin (Corsair) fans into the 4 pin hub/board slot (they’ll still fit). The 4 pin fan connection simply allows the board to control the fan speed. Since you’re using the Corsair fans with the iCUE controller (plugs into one of the USB controllers on your board), you can control fan speed via the iCUE software vs letting the board control the speed.