How to display a video on different wall sections only? Here's the thing:

We wish to display the same browser window (with 3 html5 canvas), on a wall, however, it needs to be masket only on certain points of that wall.

So, in between the wall, there will be some objects that we wish NOT to have the projector to project, like so:

------------------------------ WALL --------------------------

|(canvas1)| real flower |(canvas2)| real flower |(canvas3)|

We can connect a PC, with a graphic card with 3 outputs, OR we can have 3 graphic cards, with 3 projectors, and perhaps that will work but, does anyone know a way for doing this otherwise? For example, using one single projector?

Thank you in advance
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