Unfortunately you can't do what you are wanting to do without changing the resolution of the monitors.
So for instance, if your top monitor was 1080 pixels wide and the bottom two where each 768 which together equal 1536 pixels wide, the top monitor is just not wide enough to stretch the length of both bottom monitors. You end up with 456 extra pixels overhanging (under hanging?) the width of the top monitor.
As I said, the only way to fix this is to change the resolution of the monitors so that the top is as wide as the combined width of the bottom two or vice versa... You'll be limited by the resolutions your monitors and graphics card(s) are capable of as well.
