Why do you want to add two sound cards? Just Buy a 3 in 3 out sound card in which you will have 3 inputs along with 3 outputs.
Your problem will be solved.
i agree with techyinfo.. i think it would be best if you will try to use a soundcard that is 3on3.. we use to have that soundcard too and it works for like having a two sound system with one input. give it a try..