I have a pool of 3 drives that I use for recording. I would be nice if Plex automatically choose the drive with least space to record on instead of me setting that up per show. (sage tv does it that way.
If your drives are truly pooled and not just three drives that you use as you want then the pooling software should handle all ballasting needs. I know DrivePool does that. If the drives are just three separate drives that have nothing in common except being attached to you computer then Plex really has no way to know that the load should be balanced between them and, besides that, it is NOT Plex’s job to handle your storage for you. Each software system needs to do the job it does best.