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