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.

What if the arbitrator is colluding with the loser?

This problem is not solved via 2of3 transactions.

It can be partially mitigated by requiring multiple arbitrators (e.g. a 4/5 transaction with 3 hopefluly independent arbitrators).

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.

That's a human problem, not a Bitcoin problem.