Actually, we're both incorrect. You are correct about the description template being able to only take three character descriptions, but if anymore need to be added, after scouring the site: the Template:"FairJust2" is supposed to be used for anymore additional descriptions.
After more digging, more fields just need to be added to the FairJust template, making the FairJust2 template redundant. So the Description template can take more than three arguments, more fields just need to be added to the template.
I just asked Rab about the whole thing. Turns out the reason the Description template only has three fields is to list the most prominent characters in the image. And the FairJust2 template is redundant, accordingly.