make sure you have the correct over scan set in catalyst or ncp it always gets me when i go from a monitor to a tv. as far as the third screen it might just be a hardware bug, my laptop used to do it. If it really bothers you then there are probably some tutorials on deleting them.
Just looked into it, it seems if you are sure there isnt a third monitor or cable (by sure, remember 4k tvs like to do that split panel thing) then going into Screen Resolution on the drop down click multiple displays and it should allow you to remove it.
Will have to disconnect then remove if its stingy