Someone please correct me if I am wrong...

I think what is happening is that Win7 can only see a max of four GPUs.  You will have to move to Linux which can see up to eight.  I think that you are having the same problem that I am in that the mining software is only using one GPU from two of your three cards.  I just posted a few minutes ago and I have only one 6990 in a Win7 install but can only see one GPU for mining purposes.  If you open AMD Overdrive how many GPUs do you see?  I can see both of mine there but it is poclbm that does not see two for me.

Regardless the max that you will get working under Win7 is four so you have to make the move to Linux or take one card out. 

BTW that is a great mobo... I am using it in a rig with four 6870s.

Good luck and if you figure anything out under Win7 please post.  I will also let you know if I find an answer to my problem.