Well, without reading it, it's hard to say exactly. But in many cases, when we have a 'three's a crowd' concept in fanfiction, it often delves into the intricacies of human emotions and interactions. It might show how the three characters interact in different scenarios, how their personalities clash or complement each other. If it's set in a particular fictional world, like a fantasy or sci - fi setting, their relationships could be influenced by the rules and elements of that world as well.
I'm not sure specifically as I haven't read that particular fanfiction. However, generally, a 'three's a crowd' theme in fanfiction might involve complex relationship dynamics among three characters, such as love triangles, friendship triangles, or power struggles between three individuals within the fictional universe it's based on.