I used to run Plex on a 3b+ and it couldn't handle transcoding at a reasonable quality - jellyfin would presumably be fine as long as whatever device you are streaming from can handle your native file format (my library was x265, so I had to use the Plex desktop app because Firefox doesn't support x265 and required transcoding)

Probably not. 3Bs are pretty old.

Jellyfin happily runs on a pi, but really only for direct streaming.

You're generally better off just making the pi a samba server and connecting to it in Kodi. For jellyfin, you really do want a 7th Gen Intel or better (old 7100t-based thin clients are «chef's kiss»).