$ grep "^[^#;]" firebird.conf
ExternalFileAccess = Restrict /var/db/fb30
DefaultDbCachePages = 65000
 AuthServer = Srp, Win_Sspi, Legacy_Auth 
BugcheckAbort = 1
RemoteServicePort = 3330

SQL> set list on;
SQL> commit;
SQL> select current_connection,mon$attachment_id, mon$client_version, mon$user, mon$auth_method from mon$attachments where mon$client_version is not null;

CURRENT_CONNECTION              170
MON$ATTACHMENT_ID               170
MON$CLIENT_VERSION              LI-T3.0.0.30661 Firebird 3.0 Alpha 1
MON$USER                        SYSDBA
MON$AUTH_METHOD                 Srp

CURRENT_CONNECTION              170
MON$ATTACHMENT_ID               167
 MON$CLIENT_VERSION              WI-T3.0.0.30603 Firebird 3.0 Alpha 1 
MON$USER                        SYSDBA
MON$AUTH_METHOD                  Legacy_Auth 

# 08.08.2013: NB!! The order in which these args are is matter!
# Legacy_Auth must be FIRST if we want connect from 3.0 to 2.5:
#
#
AuthClient =Legacy_Auth, Srp, Win_Sspi