How are your two screens setup now? Are they spanned as one with like nvidia surround or amd eyefinity? If so, then don't know if you can add a 3rd and have it be separate. if your 2 current are separate, there is no reason you can't have a 3rd one be separate, providing you use the right cables and wireless hdmi transmitters only transmit what you tell it, either a clone of 1 display or a separate display. there is no way it can "only mirror" something, it simply does what the hdmi port on the back of your video card sends out.
On my GPU I have a HDMI port, Display Port,VGA, and DVI (GTX 770 GPU). I'm only making use of the HDMI's and the DVI. The thing is all the products I looked at give off the impression of mirroring the monitor witch I thought was strange,because they would take up an HDMI or USB port, witch is why I came here and figured I would ask before making a purchase and being SOL. Also thanks for such the quick response.