Three monitors work in "mirror mode" Uncheck mirror box in display settings and one monitor goes black.

Three monitors work in "mirror mode", Uncheck mirror box in display settings and one monitor goes black.
I can choose which monitor goes black; I select one and check "off" check the other two "on".
Ubuntu 16.04
64-bit
Intel® Core™ i7-2670QM CPU @ 2.20GHz × 8
Built in Display
Intel® Sandybridge Mobile
Samsung 17" DVI port
HP 17" DVI/converter/HDMI

I move the cursor and drag windows from one or more applications between two screens; I would
like to do the same with all three screens.

thank you