By default, the V.3 client tries to connect using SRP authentication
protocol. The V.1.5 server cannot respond to that. You need to set up
a client-side firebird.conf containing

AuthClient = Legacy_Auth

Take out Srp and Win_sspi. 
Don't try to configure AuthServer at the client side.
Don't copy the whole firebird.conf to the client, only
the parameters you want to configure client side.

Or pass this string in the API.

See
   https://www.ibphoenix.com/files/Authentication_FB3.pdf
Read the whole of it.  There is probably more you have missed,
such as the need for new user account records in security3.fdb.

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Helen