Assuming you've worked out how to get the live camera and the presentation into resolume, then setting up the outputs is fairly straight forward.
As i see it there are two options:
1. set the composition size to the full width of the output (for this example we'll assume 3 1920x1080 outputs not edge blended) so 5760x1080, and for the layers set the height/width to 1920/1080. Then for Layer 1 set the position X to -1920 and leave position Y at 0. For layer 3 set Position x to 1920 and again leave position Y at 0. this will position the layers to each third of the composition (so each layer takes up one full output). Then in the advanced output set up each output to take a 1920x1080 slice of the composition.
2. leave the composition at 1920x1080 and use the advanced output to route layers to each output directly. in the advanced output you can set the input to each projector directly to a Layer by-passing the composition.
Personally i'd use the first option as it allows use of the composition view as a preview of what is being sent out (where as in the second the layers would blend into each other in the composition).
Hopefully i've explained this properly, have a play around with size/poitioning layers and routing outputs.