To do a soft-edge on 3 outputs, you need the 
 - 3 video outputs: on a macPro you can connect 6 screens, or recent macBook Pro 3 full HD should be ok, if your computer can't handle 3 video connections, you can use a DualHead2Go or TripleHead2Go
 - setup in MadMapper: you have 
      - create 3 projectors dispatched in the stage (MM virtual space)
      - assign each projector to the corresponding OS screen
      - place one quad on each projector and adjust them so they overlap enough (20-30% of the projector width/height). Use a white picture at the moment. You have to set mesh warping with bezier to get nice rectangle with vertical sides of the projectors parallel.
      - then use a soft-edge calibration image applied on each quad to adjust the UVs.
You can find calibration images here: 
https://download.madmapper.com/softedge ... nMedia.zip
And a tutorial: 
https://download.madmapper.com/softedge ... t-Edge.pdf