Sorry for the really late response. Since one of the OSes is BSD I have one shared FAT32 partition mostly for basic getting-things-from-one-to-the-other stuff. Far as I know OpenBSD does not support ext4 (at least not r/w). It does support ext2.
Since all three OSes have the Nextcloud client it would have been cool to have its directory on a shared partition to reduce redundancy.
I may change things up, format it to ext2 and see if I can use it to share Documents, Music, Pictures, and Video across all three OSes. Maybe.