public void FillCmbStage()
{
    string queryString = "SELECT * FROM organizer.stages";
    using (SqlConnection connection = new SqlConnection("datasource=localhost;port = 3306;username=root;password=1234;"))
    {
        SqlCommand command = new SqlCommand(queryString, connection);
        connection.Open();
        command.CommandTimeout = 0;
        SqlDataReader reader = command.ExecuteReader();
        comboBox3.Items.Clear();
        while (reader.Read())
        {
            comboBox3.Items.Add(reader[0].ToString());
        }
        reader.Close();
    }
}