My basic understanding is that all trades must involve the player whose turn it is.
Alice, whose turn it is, has ore and wants wheat. Bob wants ore and offers brick, which is not acceptable to Alice. Carol wants brick, and is willing to give wheat for it. So a three way trade would have Alice sending ore to Bob, Bob sending brick to Carol, and Carol sending wheat to Alice. Put another way, Alice gives ore to Bob and receives wheat from Carol.
Would such a trade be permitted under the rules? Looked at one way, Alice has dealt with both Bob and Carol. Looked at another way, there was a Bob to Carol leg (Bob sends brick to Carol) that did not involve Alice.
That is unless Alice first trades ore for brick with Bob, then brick for wheat with Carol. Can these two trades be made "simultaneous" through an "escrow" type arrangement? Or is this feature only reserved for say, "house rules?"