Can't grant privileges to "root" when it has no password in MariaDB
"If the NO_AUTO_CREATE_USER SQL_MODE is set, users can only be created with a CREATE USER statement. In this case GRANT will produce an error when the specified user does not exist." -- https://mariadb.com/kb/en/mariadb/grant/

If that is the case, then the solution is probably:

CREATE USER 'root'@'192.168.3.%';
GRANT ... (as you tried)


"Can't find any matching row..." means that there was no row with both user='root' and host='192.168.3.%'.  You were coming in with user='root' and host='localhost' (or something like that).