cgminer doesn't recognize my third 7970

Hello,
I have just plugged a new third 7970 card on my litecoin mining rig running ubuntu 13.04 and cgminer 3.1.0 but it seems that cgminer doesn't recognize it...

When I run cgminer I can only see GPU 0 and GPU 1 accepting blocks with my pool.

When I launch cgminer -n
I have this result:

Platform 0 devices 2
0 Tahiti
1 Tahiti
Failed ADL_Adapter_ID_Get. Error 10
This error says the device is not enabled
GPU 0 hardware monitoring enabled
GPU 1 hardware monitoring enabled
2 GPU devices max detected...

But I can't see the third GPU in cgminer.....

When I launch aticonfig --list-adapters:

*0. 01:00.0 AMD Radeon HD 7900 Series
1. 06:00.0 AMD Radeon HD 7900 Series
2. 07:00.0 AMD Radeon HD 7900 Series
* - Default adapter

Is there any tricks to enable it, such as dummy plugs, or manipulate xorg.conf?
I think I'm missing a very simple step, but I don't know which... Huh

Thanks for help. Grin