how do I make all 3 cards available

I have 3 terratec EWS88mt sound cards. each has 8 inputs and 8 outputs (they are synced via hardware). I know it’s possible to make them all show up, but it seems that I can only see 2 of them, and can only choose 1 at a time…does anyone know how to set them up? If it matters, they are envy24 based…

Thanks for any help!