For normal usage I used the right one (which was slightly bigger than the left one) for web navigation, the center for other programs, video/streaming, and programming IDEs, the left for system audio mixer (jack_mixer, I always have jack running), shells and system monitoring; for audio I used the central display for the Ardour main track window, the left for the mixer strip/qjackctl/qsampler/plugin GUIs, with jack connections in the background with Patchage, while the right one was for other programs (music notation or sequencing, Hydrogen, Renoise, scripting, etc).
Some months ago the right display broke down and, while I think I'll replace it someday, the setup remained pretty similar, with programs that usually run on the right splitted between central and left, according to the size needed. I started using virtual desktops much more than in the past (I use fluxbox with 4 virtual desktops), so the second desktop is usually dedicated to music players, social networks and chat apps, with the third and fourth used as "backup" spaces when I need to access to a program easily without activating it from the taskbar, for example when using virtualization softwares (I configured the shortcut Win key+mousewheel to cycle between virtual desktops).