You get that error because Jaybird 3 (just like Jaybird 2.2) doesn't
support Firebird 3 wire encryption, by default Firebird 3 requires
encryption and will reject connections that don't ask for encryption.
You need to change firebird.conf and set WireCrypt = Enabled (default
value is Required). See also
https://github.com/FirebirdSQL/jaybird/wiki/Jaybird-and-Firebird-3 That
wiki is more about Jaybird 2.2; I will update it to include a Jaybird 3
section, and add a section to the release notes to mention this
explicitly as well.

Mark