They mention "FIDO® Certified* security keys", this presumably means physical keys only, and not soft keys like the ones that keepassxc/bitwarden provides? If so that might be too much of a hassle for me. I care about my security, but I don't care enough to drop $100 on 3 separate security keys, and finding 3 separate places to keep them secure.

You need two keys, not three.

But yes I wish you could use one hardware key as backup and one software key for day-to-day usage, or at least the security key in a trusted device (up to you to have a circular dependency to your main device or not).

It does not help you when a trusted device is stolen, the yubikeys can be disabled if they unlock your phone or device