con.connect(function(err) {
    if (err)
    {throw err }
    con.query("SELECT id FROM Table_1 WHERE username = xyz AND password = aaa",
        function (err, result) {
           if (err) {throw err};
           con.query("SELECT table_3_id FROM table2 WHERE Table_1_id = "+(result)+")",
                  function(/*bla-bla*/){

                  };
                });
    }
);