How to display a video on different wall sections only?
With even the most simple projector, if part of the image is black, then nothing will be projected at that point, so why not set up your image to be: 


  |(canvas1)| blank space |(canvas2)| blank space |(canvas3)|


Deciding whether that space should be black, white or some other colour is down to you - do you want your real flowers lit? If so, make it a white space