You can quote Wikipedia articles as long as you want. Nothing to do with graphics pipelines, we are talking about the rendering engine of FS2020 and it’s output.
The gpus do nothing on their own, they need the software to tell them what to do. FS2020 has a single render pipeline for whatever it displays across how many outputs.
Your 2nd and 3rd outputs are not magically being split from everything else and left to each GPU in turn to render, FS is not splitting different views off and sending each GPU only the relevant objects and data they need and hoping they all render each nicely and in sync with each other.
It’s pushing all that’s called for out to the primary GPU.
It’s why SLI was created, and it’s modern implementation nvlink, to share rendering and vram between multiple GPUs rather than just take advantage of individual cards. But you need nVidia for that.