I would spend some cash and rent a video mixer, doing an ABC switch for switching live video is going to drive you up a tree, and the projectors/TV as well. Each time you switch video the projecter will have to re-sync to that source, you WILL get video jumps, and possible blue screens. Remember, ABC switches are just simple contact closures. I am also concerned that you are going to lose your video signal with all that splitting and cable runs, might want to look into some composite amplifiers. I would also take a long and hard look at the wax paper screen thing, paper products have high and low spots, I don't think its going to look the best for an RP screen.
You might also want to attempt to go to PC based content instead of DVD, I think you might be much happier. You can get 3 pc's, link them together using a program called synergy,
http://synergy2.sourceforge.net/, and be able to cue them all at the same time. Either that, or put the same content on all 3 DVD players, and only cue the one you need at the time. So, DVD 2 and 3 would have the content that would be played from DVD 1 when you want the same content on all 3. That would take out a lot of the difficulty.
Just a side note, real world you would use a watchout system to do this, this is what that technology was made for. I would also be interested in seeing what the video wall thing turns out looking like. Also, keep in mind "video black", no matter what you send those TV's and projectors, they will still be spitting out light, so blackouts will not be possible in this production without dousers on everything.