No Continuous desktop across Multi Monitors.

Hi - I have three monitors - the center has the menu bar - the mouse moves continuously from one to another - all fine. But when I slide a graphic from one screen to the other it slides fine (with ghosting on one screen) but when I release the mouse - the graphic cuts off one screen. can't figure out why I can't configure a continuous desktop across screens. I basically have three independent screens that are connected by sliding but with no continuous graphics.

2012 Mac Pro cheesegrater - three 21 inch monitors - Mac OS Sierra