X11 middle button emulation

Some X11 application like ImageMagick's display, need the middle button for certain functionalities.

To enable the 3rd button emulation for the Trackpad under X11, go to the X11 application and check:  

      X11> preferences > Input > Emulate tree button mouse.

The third button click is then emulated with: 

      Option(alt) - Command(⌘) - Right Click