By default, FX3 doesn't have proper 21:9 support, it pillarboxes 16:9 in landscape and letterboxes 9:16 in portrait orientations via the cabinet mode advanced settings.
You can fill a 9:21 ultra..tall portrait resolution by using Windows' portrait orientation instead of the game's cabinet rotation setting, but the results are a cropped / zoomed to fill 9:16, and with pinball tables that means the sides of the playfields are missing.
Here's both methods side-by-side:
https://i.imgur.com/MqRSGv8.jpgI messed about with cheat engine trying to find some value to zoom the camera out a bit but got nowhere after some time. If anyone wants to give it a shot or has a better approach, the game base is free, including this Fish Tales table (other tables have to be purchased). I'd recommend using the Classic Mode with Arcade or Tournament physics setup if you try to actually play it!
The Williams Pinball Volume 1 pack with real tables and a more realistic physics simulation just launched for Zen Pinball FX3 the other day. This may wind up being bigger than Pinball Arcade which lost the Bally / Williams licenses if Zen keeps supporting it, but the default views and camera options on all screens really needs some work, especially in portrait orientation. Brought this up to the devs during the beta and nothing has improved since.