So, this is probably the best we can do economically as a system that could run parallel to a daytime exhibit.
As a dedicated exhibition, there are some improvements that could be made:
Firstly, blackout curtains could be hung inside the room to stop light reflected around inside the room from coming through the screen. The brightness would remain the same but as you can see from the image, the real problem is not so much the bright areas as that the blacks are just not black enough. (Actually, it might be possible to do this for a foldaway system too - just have black blinds that come down over the walls as well as a white one for the window)
For more brightness, we could use all three projectors, each one lighting up one window pane and displaying a portrait format image. With each running on a separate, out of sync timer, this could make for an interesting effect and would also remove the issue of the window frame interrupting the composition of images.