Why cant I enable my 3rd monitor in Win7 control panel

[zappafan1]

Hello,
I am trying to connect and use a second external monitor thru the DP port on my laptop docking station with a DP to usb adapter. In the control panel the 3rd monitor is connected but greyed out. Advice anyone on how to activate and extend to this 3rd screen??

 

[bignastyid]

What model laptop do you have? Most laptops only support 2 displays. I have never heard of a displayport to USB adapter.