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.
Unfortunately, there's no really good way of establishing that two pseudonyms are not held by the same person, in a decentralized system. Doubly so when legal enforcement isn't available. Of course abuse is harder the more arbitrators you incorporate, but expense and difficulty are higher as well.
