Worth noting that burgers has crossposted this to LFPF and got a very cool reply suggesting the use of a 3-way compact prism for beamsplitting. Such an arrangement should be on a similar order of depth as a traditional SLR mirror, which means you could maybe use retrofocus SLR lenses (RB67) to get normal/wide views.
Anyway, I suspect it'd be a good idea to implement decent tricolour printing (from consecutive shots on a rollfilm back) before building a camera like this. To me, the camera looks like a minor mechanical challenge of a similar order of complexity as a traditional camera build, just with a few more parts. The camera is of no use unless you can print (scan?) the separations, and the printing I think will be a real challenge.