Assigning multiple sound card inputs to different input channels

[Trakzuk Radio]

Right. Tray again. I have a PC with 1 internal sound card and 3 external USB sound cards. Mix Bus 5 can see all the input devices in the 'Audio/Midi Setup' window, but selecting any one of them simply causes the one selected to be applied to all 4 input channels. I need to assign each one of the sound cards to each one of the input channels on the mixer.