Sorry, I do not know the situation with TWRP for your Pixel device. But check to TWRP.me (official TWRP site), do they have TWRP for your model and your Android versionHow do I uninstall? I can't access the app to uninstall it, and the uninstallation instructions just say that if you can't do that, "flash it like any flashable zip", and I have no idea how to do that. I know TWRP stopped working, is that back? The TWRP thread in the Pixel 3 forum hasn't been updated in years. Or can I just flash it with fastboot?
In the worst case (no TWRP or other custom Recovery) and you cannot open Magisk app, you can always (I know, it's pain in the ass) reinstall your ROM with Factory resetting. That would also fully uninstall Magisk by also deleting all its stuff (modules, database, etc)
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If you are good with using the Terminal emulator and shell commands, you can avoid Factory resetting
To see if that apply, open your Terminal emulator and execute:
su
magisk -c
If it prints your version of Magisk installed, we can go for
But if it cannot find su, whatever - then sorry, I will not second in that direction further (even if it is 'just' about which Terminal emulator to use or how)