Nor have I; I think it is just what the developer of tree has chosen to call file descriptor 3, rather than being a wider convention or standard thing provided by the environment.

> As of version 2.0.0, in Linux, tree will attempt to automatically output a compact JSON tree on file descriptor 3 (what I call stddata,) if present

https://github.com/Old-Man-Programmer/tree/blob/d501b58ff9cb...