There is no such thing. In a 2 out of 3 transaction, 2 out of 3 parties have to agree for the transaction to happen. Thus, if the winner agrees and the arbitrator also agrees, the transactions is conducted. It's a cryptographically secure agreement.
It can be partially mitigated by requiring multiple arbitrators (e.g. a 4/5 transaction with 3 hopefluly independent arbitrators).