The problem with your suggestion will be performance - VAS usage
I haven't suggested to run everything on a single PC, but to run at least the visuals on a single PC. In this case, there will be no difference whatsoever on VAS usage, because the memory-hungry airplane (I guess this was your point about VAS usage) will run on a different PC.
and FPS issues will undoubtably arise. the resolution of 3 monitors on 1 card coupled with all my add-ons; NOT a cat in hells chance!
As I've said, FSX is generally NOT fill-rate limited, it's usually way more cpu-limited instead. Not being fill-rate limited means you can run a video card that has enough fill-rate to drive 3 monitors, without starting to notice fps decrease BECAUSE of FSX. And of course, the two examples of cards I've listed can do that, because since FSX doesn't benefit from a dual GPU, and the "wasted" horsepower that FSX can't use anyway, can be used to support 3 monitors with no fps decrease.
On top of that, by not networking the visuals, you don't risk seeing those micro-stutters due to the network connection and latency, which are more and more visible, the larger your screen will be.
That, without even mentioning that with 3 networked PCs for the visuals, you will constantly have to check for explicit network compatibility for many add-ons (ANYTHING that draws things on screen based on its own logic will have issues) a problem that doesn't exists if the visual are generated by a single system.