[michaelbsc]

...I think that these Curtis and Devin one-shots used half-silvered mirrors, but I know you're set on the beam splitter idea and that indeed could be a great route to take....

That's all a beam splitter is, a half silvered mirror. Take a look at the Edmund's Scientific stuff.

... I've just read numerous accounts in old books about how tight the tolerances need to be to ensure that the 3 negatives are in the identical plane of focus.

That's why I think building to tolerance is a tough plan. You have to build with the plan to shim as required, and do it empirically.

I'm sure someone here knows how, but truth be told I'm certain I cannot calculate how the wavefront of the image changes with some parts going through one sheet of glass, and other parts going through two sheets. I know that affects the physical distance because of the transmission medium differences in the paths. But not in any way that I can predict.

So, you build adjustment into the device, and you tweak!