You can't use 3 GPUs I'm afraid, the devs have said mGPU currently only supports 2. There are already 2 in the 7990 so you can't combine that with the 960.
As DX12 mGPU is different I think you actually have to have CF turned off for it to work correctly. Then you boot up in dx12, tick mGPU in the options, restart the game and for it to work...Though as yours is two cards in one this may cause problems? I don't know about that.
