I don’t think that anyone is frustrated with you; rather, we’re trying to help.
As I mentioned in the original thread on the After Effects forum, I don’t think you’re asking the right questions upfront. Before you worry about the After Effects setup, you need to sort out the physical setup, because this will determine what you need to do in AE.
It sounded to me like you were currently planning on using three 4:3 projectors side by side. That’s not a good solution for a seamless widescreen display. Instead, you want to overlap the projectors slightly, 15-25% with an edge blend (with each image feathered on the sides in the overlap area), to make a seamless display. This will result in something narrower than a 12:3 display.
Once you have your projection finalized, then you can worry about how to design the content for it, but not before. I’d suggest that the correct resolution will be neither 2160×480 nor 3072×768. You will want to create your media at exactly the resolution of the blended projection display, which will be 768 pixels tall. We can’t suggest a horizontal resolution yet, because the question of projection and the amount of overlap is still up in the air.
I ask again: are you alone on this, or do you already have someone helping you with the projection? Who’s responsible for physically setting up the projectors? Are you working with an AV company or a production company? If there is someone, you need to talk to them about how they are planning on setting up projection. If not, you should try to get some help on this, because you really need the edge blend in projection to make this work well. Are there any other visual sources besides your video? If not, will the projectors have the edge-blending feature built-in, or will you need to manage the edge-blend upstream of the projection? How did you arrive at the solution you’re currently considering?
There are a lot of considerations, but this is certainly not an impossible task. However, you might need some additional resources beyond what you are planning now to make this event a sucess.
Walter Soyka, Principal
Keen Live, Inc.
Presentation, Motion Graphics & Widescreen Design
RenderBreak: A Blog on Innovation in Production