Before they enter upon a reference the two arbitrators shall appoint a third arbitrator. Should they fail to appoint such a third arbitrator within thirty days of the appointment of the respondent's arbitrator then either of them or either of the parties may apply to the appointor for the appointment of the third arbitrator. The three arbitrators shall decide by majority. If no majority can be reached the verdict of the third arbitrator shall prevail. He shall also act as chairman of the tribunal.