Only one arbitrator is necessary to hear a case. Sometimes an arbitration agreement (or the rules of an association that are incorporated by reference in the agreement) will provide for three or more neutral arbitrators. (There are always an odd number of arbitrators so that a decision can be made by a majority.)
In some cases a "tripartite" arbitration panel is used. In tripartite arbitration each party picks its own party-appointed arbitrator and the two party-appointed arbitrators choose a neutral third arbitrator.